ARM: dts: dra7-evm: Add mmc2 node for eMMC support

Add mmc2 dt node to dra7-evm board
and model eMMC vcc as fixed regulator.

Signed-off-by: Balaji T K <balajitk@ti.com>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>