ARM: dts: omap5: board-common: fix wrong SMPS6 (VDD-DDR3) voltage

DDR3L is usually specified as

	JEDEC standard 1.35V(1.28V~1.45V) & 1.5V(1.425V~1.575V)

Therefore setting smps6 regulator to 1.2V is definitively below
minimum. It appears that real world chips are more forgiving than
data sheets indicate, but let's set the regulator right.

Note: a board that uses other voltages (DDR with 1.5V) can
overwrite by referencing &smps6_reg.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>