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>