serial: of_serial: Handle fifo-size property

This will reduce the need for extra types in 8250.c just
in case the fifo size differs from the standard.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>