tty: serial: Makefile: move kgdb to be initialized last

fsl_lpuart cannot be used with kgdb because the kgdb
initcall is called before the driver's init.
Move kgdb to be initialized after all serial drivers
have been inited.

Signed-off-by: Nicolae Rosia <nicolae_rosia@mentor.com>
Signed-off-by: Stefan Golinschi <stefan.golinschi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>