ARM: OMAP3/OMAP4: PRM: add prm_features flags and add IO wakeup under it

prm_features flag will contain SoC specific feature enabler flags. Initially
IO wakeup is added under this. Helps to get rid of runtime cpu_is_X checks.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>