imx: support i.MX8QM MEK board

Add i.MX8QM MEK board support.
Included a basic dts, enabled SPL FIT

Boot log as below:
U-Boot SPL 2019.01-rc1-00029-gf002213219 (Dec 24 2018 - 10:28:30 +0800)
Normal Boot
Trying to boot from MMC2_2

U-Boot 2019.01-rc1-00029-gf002213219 (Dec 24 2018 - 10:28:30 +0800)

CPU:   NXP i.MX8QM RevB A53 at 142933 MHz

Model: Freescale i.MX8QM MEK
Board: iMX8QM MEK
Build: SCFW 9330215b
Boot:  SD1
DRAM:  6 GiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1
Loading Environment from MMC... *** Warning - bad CRC, using default environment

In:    serial@5a060000
Out:   serial@5a060000
Err:   serial@5a060000
Net:
Error: ethernet@5b040000 address not set.
eth-1: ethernet@5b040000
Hit any key to stop autoboot:  0

Signed-off-by: Peng Fan <peng.fan@nxp.com>