serial: 8250: Add support for big-endian MMIO accesses

Add cases for UPIO_MEM32BE wherever there are currently cases handling
UPIO_MEM32.

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