OMAP2+: PM: Voltage: allow software margin per OPP

Allow per OPP software margin to be defined in OPP table
for the voltage definitions.

Note: OMAP4460 has currently recommended 10mV margin to be added.

This also introduces /sys/kernel/debug/voltage/*/curr_margin_volt
to show the current voltage margin used.

Change-Id: Id49185ffb224d03ac05a41780ad35b1c7a20f4a0
Signed-off-by: Nishanth Menon <nm@ti.com>

Conflicts:
	arch/arm/mach-omap2/omap_opp_data.h
	arch/arm/mach-omap2/opp4xxx_data.c