arm: imx: add i.MX6SLL EVK board support

Add i.MX6SLL EVK board support.
1. Add imx6sll-evk device tree.
2. Enable SDHC/I2C/UART.
3. Enable REGULATOR/PMIC/I2C/GPIO/SDHC/PINCTRL driver.

Boot Log:
U-Boot 2016.11-00127-gc635871-dirty (Nov 24 2016 - 13:28:19 +0800)

CPU:   Freescale i.MX6SLL rev1.0 at 792MHz
CPU:   Commercial temperature grade (0C to 95C)Reset cause: POR
Model: Freescale i.MX6SLL EVK Board
Board: MX6SLL EVK
DRAM:  2 GiB
i2c bus 0 at 35258368, no gpio pinctrl state.
PMIC: PFUZE100! DEV_ID=0x10 REV_ID=0x21
MMC:   FSL_SDHC: 0, FSL_SDHC: 1, FSL_SDHC: 2
In:    serial
Out:   serial
Err:   serial
Net:   CPU Net Initialization Failed
No ethernet found.
Hit any key to stop autoboot:  0

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>