OMAP DSS normally gets power from VCXIO on OMAP4. Add configuration for
this into twl-common.c

Mark VCXIO as always_on, as VCXIO is used by multiple components,
including the MPU, and turning it off when DSS doesn't need it would
lead the device to halt.
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>