ARM: OMAP2+: PRCM: cleanup some header includes

Some of the includes are totally unnecessary, remove some others in
preparation to make the PRCM its own driver.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
[paul@pwsan.com: updated to apply; fixed build error on OMAP2xxx-only configs]
Signed-off-by: Paul Walmsley <paul@pwsan.com>