doc: arch: Convert README.arm64 to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

doc/arch/arm64.rst

@@ -0,0 +1,59 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+ARM64
+=====
+
+Summary
+-------
+The initial arm64 U-Boot port was developed before hardware was available,
+so the first supported platforms were the Foundation and Fast Model for ARMv8.
+These days U-Boot runs on a variety of 64-bit capable ARM hardware, from
+embedded development boards to servers.
+
+Notes
+-----
+
+1. U-Boot can run at any exception level it is entered in, it is
+   recommened to enter it in EL3 if U-Boot takes some responsibilities of a
+   classical firmware (like initial hardware setup, CPU errata workarounds
+   or SMP bringup). U-Boot can be entered in EL2 when its main purpose is
+   that of a boot loader. It can drop to lower exception levels before
+   entering the OS.
+
+2. U-Boot for arm64 is compiled with AArch64-gcc. AArch64-gcc
+   use rela relocation format, a tool(tools/relocate-rela) by Scott Wood
+   is used to encode the initial addend of rela to u-boot.bin. After running,
+   the U-Boot will be relocated to destination again.
+
+3. Earlier Linux kernel versions required the FDT to be placed at a
+   2 MB boundary and within the same 512 MB section as the kernel image,
+   resulting in fdt_high to be defined specially.
+   Since kernel version 4.2 Linux is more relaxed about the DT location, so it
+   can be placed anywhere in memory.
+   Please reference linux/Documentation/arm64/booting.txt for detail.
+
+4. Spin-table is used to wake up secondary processors. One location
+   (or per processor location) is defined to hold the kernel entry point
+   for secondary processors. It must be ensured that the location is
+   accessible and zero immediately after secondary processor
+   enter slave_cpu branch execution in start.S. The location address
+   is encoded in cpu node of DTS. Linux kernel store the entry point
+   of secondary processors to it and send event to wakeup secondary
+   processors.
+   Please reference linux/Documentation/arm64/booting.txt for detail.
+
+5. Generic board is supported.
+
+6. CONFIG_ARM64 instead of CONFIG_ARMV8 is used to distinguish aarch64 and
+   aarch32 specific codes.
+
+
+Contributors
+------------
+   * Tom Rini            <trini@ti.com>
+   * Scott Wood          <scottwood@freescale.com>
+   * York Sun            <yorksun@freescale.com>
+   * Simon Glass         <sjg@chromium.org>
+   * Sharma Bhupesh      <bhupesh.sharma@freescale.com>
+   * Rob Herring         <robherring2@gmail.com>
+   * Sergey Temerkhanov  <s.temerkhanov@gmail.com>

doc/arch/index.rst

@@ -6,5 +6,6 @@ Architecture-specific doc
 .. toctree::
    :maxdepth: 2
 
+   arm64
    mips
    x86