arm64: zynqmp: Enable SPL_SEPARATE_BSS by default

BSS section was all the time separated for SPL but this symbol wasn't
enabled. It is necessary to have it enabled for OF_SEPARATE
configuration where DTB is appended to u-boot with DTB.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/Kconfig

@@ -928,6 +928,7 @@ config ARCH_ZYNQMP
 	select OF_CONTROL
 	select SPL_BOARD_INIT if SPL
 	select SPL_CLK if SPL
+	select SPL_SEPARATE_BSS if SPL
 	select SUPPORT_SPL
 	imply BOARD_LATE_INIT
 	imply CMD_DM