omap: UART: fix wakeup registers for OMAP24xx UART2

On OMAP24xx, UART2 WKEN and WKST registers are in PM_WKEN2_CORE and
PM_WKST2_CORE respecitvely.  Fix the OMAP2 register init to use the
correct registers on OMAP24xx.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>