ARM: dts: dra7-evm: Rename mmc2_3v3 supply to evm_3v3_sw

Use the name for the supply as it is in the schematics since the same
supply is used for other peripherals than MMC2, like audio.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>