ARM: DRA7: PRM: add voltage processor check behind a prm_feature flag

This is done in attempt to get rid of cpu_is_X calls from the PRM core.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>