imx: mx6slevk: add SPL support

Add SPL boot support for mx6slevk board.
1. Introduce a configuration file mx6slevk_spl_defconfig.
2. i.MX6SL has same DRAM space with i.MX6SX, need to change SPL DRAM SPACE.
3. Include imx6_spl.h and related SPL macro in mx6slevk.h.
4. select SUPPORT_SPL for TARGET_MX6SLEVK.
5. Add SPL board code to do related initialization.

Boot Log:

U-Boot SPL 2015.07-00544-g1594a76 (Aug 17 2015 - 01:56:59)
reading u-boot.img
reading u-boot.img

U-Boot 2015.07-00544-g1594a76 (Aug 17 2015 - 01:56:59 +0000)

CPU:   Freescale i.MX6SL rev1.2 996 MHz (running at 396 MHz)
CPU:   Commercial temperature grade (0C to 95C) at 50C
Reset cause: POR
Board: MX6SLEVK
I2C:   ready
DRAM:  1 GiB
PMIC:  PFUZE100 ID=0x10
MMC:   FSL_SDHC: 0, FSL_SDHC: 1, FSL_SDHC: 2
*** Warning - bad CRC, using default environment

In:    serial
Out:   serial
Err:   serial
Net:   FEC [PRIME]
Hit any key to stop autoboot:  0

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>

include/configs/imx6_spl.h

@@ -61,7 +61,7 @@
 #define CONFIG_SPL_LIBDISK_SUPPORT
 #endif
 
-#if defined(CONFIG_MX6SX) || defined(CONFIG_MX6UL)
+#if defined(CONFIG_MX6SX) || defined(CONFIG_MX6UL) || defined(CONFIG_MX6SL)
 #define CONFIG_SPL_BSS_START_ADDR      0x88200000
 #define CONFIG_SPL_BSS_MAX_SIZE        0x100000        /* 1 MB */
 #define CONFIG_SYS_SPL_MALLOC_START    0x88300000

include/configs/mx6slevk.h

@@ -11,6 +11,13 @@
 
 #include "mx6_common.h"
 
+#ifdef CONFIG_SPL
+#define CONFIG_SPL_LIBCOMMON_SUPPORT
+#define CONFIG_SPL_MMC_SUPPORT
+#define CONFIG_SPL_FAT_SUPPORT
+#include "imx6_spl.h"
+#endif
+
 #define MACH_TYPE_MX6SLEVK		4307
 #define CONFIG_MACH_TYPE		MACH_TYPE_MX6SLEVK