serial: omap: implement set_wake

This has been missing from OMAP UART driver
for quite a while and it's simple enough
to implement it.

Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>