i.MX6UL: isiot: Add eMMC boot support

i.MX6UL: isiot: Add eMMC boot support

Boot from eMMC:
--------------
U-Boot SPL 2017.01-00314-gd0cd9cd-dirty (Jan 25 2017 - 13:25:27)
Trying to boot from MMC2

U-Boot 2017.01-00314-gd0cd9cd-dirty (Jan 25 2017 - 13:25:27 +0100)

CPU:   Freescale i.MX6UL rev1.1 528 MHz (running at 396 MHz)
CPU:   Industrial temperature grade (-40C to 105C) at 36C
Reset cause: POR
Model: Engicam Is.IoT MX6UL eMMC Starterkit
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

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