OMAP: UART: use non-locking versions of hwmod enable/idle functions

Since the UART enable/idle is done during the idle path (with
interrupts disabled), use the non-locking versions of the hwmod
enable/idle functions.

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