ARM: OMAP3+: PRM: remove prm_get_reset_sources declaration from headers

There is no implementation for this anywhere, so remove it from the header
files also.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
[tony@atomide.com: updated to apply]
Signed-off-by: Tony Lindgren <tony@atomide.com>