omap2+: pm_bus: make functions used as pointers as static

omap_pm_runtime_suspend and omap_pm_runtime_resume are used
as function pointers and does not really need to be exposed
to the world.

Fixes sparse warnings:
arch/arm/mach-omap2/pm_bus.c:23:5: warning: symbol 'omap_pm_runtime_suspend' was not declared. Should it be static?
arch/arm/mach-omap2/pm_bus.c:40:5: warning: symbol 'omap_pm_runtime_resume' was not declared. Should it be static?

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>