OMAP: UART: don't do automatic bus-level suspend/resume

Since the omap_device for UART is currently managed inside the idle
path itself,  don't let the bus-level code suspend/resume the UART.

To prevent this, pm_runtime_get() is used when preparing for suspend
and pm_runtime_put() is used when finished with suspend.

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