imx: add i.MX8MP EVK board

Add basic i.MX8MP EVK board support

U-Boot SPL 2020.01-rc4-00388-gb1bf40c0ae-dirty (Dec 30 2019 - 17:55:33 +0800)
power_pca9450b_init
DDRINFO: start DRAM init
DDRINFO:ddrphy calibration done
DDRINFO: ddrmix config done
Normal Boot
Failed to find clock node. Check device tree
WDT:   Not found!
Trying to boot from BOOTROM
image offset 0x8000, pagesize 0x200, ivt offset 0x0

U-Boot 2020.01-rc4-00388-gb1bf40c0ae-dirty (Dec 30 2019 - 17:55:33 +0800)

CPU:   Freescale i.MX8MP rev1.0 at 1000 MHz
Reset cause: POR
Model: NXP i.MX8MPlus EVK board
DRAM:  6 GiB
MMC:   FSL_SDHC: 1, FSL_SDHC: 2
Loading Environment from MMC... OK
In:    serial
Out:   serial
Err:   serial
Net:   No ethernet found.
Hit any key to stop autoboot:  0
u-boot=> mmc list
FSL_SDHC: 1 (SD)
FSL_SDHC: 2

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