ARM: OMAP2+: PRM: initialize en_uart4_mask and grpsel_uart4_mask

In the case where has_uart4 is false, en_uart4_mask and grpsel_uart4_mask
are not initialized and so any garbage value is being logically or'd into
the write of PM_WKEN and OMAP3430_PM_MPUGRPSEL.  Fix this by initializing
these masks to zero.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>