ARM: dts: imx6ul: phycore: Add eMMC at usdhc2

The phyCORE-i.MX 6UL/ULL now can have eMMC instead of the NAND flash
memory. Add the eMMC node and disable it by default so it can be enabled
in case it is populated.

Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>