OMAP3: serial: Fix uart4 handling for 3630

This patch makes the following:
 - Adds missing wakeup padding register handling.
 - Fixes a hardcode to use PER module ONLY on UART3.

Signed-off-by: Sergio Aguirre <saaguirre@ti.com>
Signed-off-by: Govindraj.R <govindraj.raja@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>