imx6: icorem6_rqs: Add eMMC boot support

Boot from eMMC:
--------------
U-Boot SPL 2017.01-00318-g8e243f8 (Jan 26 2017 - 11:53:21)
Trying to boot from MMC2

U-Boot 2017.01-00318-g8e243f8 (Jan 26 2017 - 11:53:21 +0100)

CPU:   Freescale i.MX6D rev1.2 at 792 MHz
Reset cause: POR
Model: Engicam i.CoreM6 Quad/Dual RQS Starter Kit
DRAM:  512 MiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1
*** Warning - bad CRC, using default environment

In:    serial
Out:   serial
Err:   serial
switch to partitions #0, OK
mmc1(part 0) is current device
Net:   No ethernet found.
Hit any key to stop autoboot:  0
Booting from mmc ...
switch to partitions #0, OK
mmc1(part 0) is current device

Cc: Stefano Babic <sbabic@denx.de>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>