ARM: dts: dra7-evm: Correct the vmmc-supply for mmc2

On DRA75x EVM, MMC2 vdd/ios are connected to a common supply fixed at 1.8V
not 3.3V

Fixes: 6cf02dbb ("ARM: dts: dra7-evm: Add mmc2 node for eMMC support")
Signed-off-by: Ravikumar Kattekola <rk@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>