Engicam Is.IoT MX6UL has separate module for eMMC, so add emmc dts
file for imx6ul-isiot.dtsi, usdhc2 node represent eMMC.

dmesg:
-----
mmc1: SDHCI controller on 2194000.usdhc [2194000.usdhc] using ADMA
mmc1: new DDR MMC card at address 0001
mmcblk1: mmc1:0001 M62704 3.53 GiB

Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>