serial: 8250_of: Add basic PM runtime support

66AK2G UART instances are not apart of the ALWAYS_ON power domain.
Therefore, pm_runtime calls must be made to properly insure the appropriate
power domains needed by UART are on. Keep legacy clk api calls since other
users of this driver may not support PM runtime.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>