ARM: dts: dra7-evm: Model EVM_3V6 regulator

On TI's DRA74x EVM, EVM_3V6 is connected is connected to the VBAT line
of the wilink card. Model it here so that it can be used while adding
wilink8 WLAN support.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>