ARM: OMAP4: PRM: make omap4_prm_read/write_inst_reg calls static

These are not (and should not be) used by anybody outside the PRM
driver itself.

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>