TTY: serial: convert drivers/tty/serial/* to use module_platform_driver()

This patch converts the drivers in drivers/tty/serial/* to use the
module_platform_driver() macro which makes the code smaller and a bit
simpler.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jamie Iles <jamie@jamieiles.com>
Cc: Yoichi Yuasa <yuasa@linux-mips.org>
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>