ARM: dts: dra7-evm: Add fixed regulator to be used by aic3106's DVDD

TPS77018DBVT is used to create 1.8V from avm_3v3_sw's 3.3V connected to
aic3106's DVDD.

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