ARM: OMAP2+: Voltage: Move the omap_volt_data structure to plat

Move the omap_volt_data structure from mach-omap2/ directory
to arch/arm/plat-omap/include/plat/ so that it is accessible
from both mach-omap2 and drivers directories.

Signed-off-by: J Keerthy <j-keerthy@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>