ARM: OMAP2+: PRCM: Clear reset sources after boot

According to TRMs the reset source flags have to be cleared by
software, in another case the flags stay after reboot. So clear
them to avoid the flag gathering.

Change-Id: Ia4a5dadd67a2459f6b644d629768d7d51a365efb
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com