serial: 8250: allow specifying iomem size in addition to address

This adds a mapsize field to struct uart_port to be used in
conjunction with mapbase. If set, it overrides whatever value
serial8250_port_size() would otherwise report.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>