serial: of: set port iomem size from devicetree

Make the 8250 driver reserve exactly the mmio region specified
in the devicetree. This prevents conflicts between UPIO_AU type
UARTs and other devices mapped closer than the default size of
0x1000 (or 0x100 for RT2880).

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