xtensa: xtfpga: fix serial port register width and endianness

Serial port is attached to XTFPGA boards as native endian device, mark
it as such in DTS and pass correct endianness in platform data.
Set register width in DTS to 4, this way it matches the platform data
and works correctly on big-endian CPUs.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>