serial: of_serial: Support big-endian register accesses

If the device node has a "big-endian" property and 32-bit registers, tell
the serial driver to use UPIO_MEM32BE instead of UPIO_MEM32.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>