serial: meson: make use of uart_port member mapsize

Member mapsize of struct uart_port is meant to store the resource size.
By using it we can get rid of meson_uart_res_size().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>