serial: omap-serial: Add support for kernel debugger

The kgdb invokes the poll_put_char and poll_get_char when communicating
with the host. This patch also changes the initialization order because the
kgdb will check at the very beginning, if there is a valid serial
driver.

Signed-off-by: Cosmin Cojocar <cosmin.cojocar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>