xtensa: xtfpga: fix earlycon endianness

Serial port is attached to XTFPGA boards as native endian device, now
that earlycon parameter parser understands mmio32native put it into
earlycon kernel parameter. This makes early console functional on both
little- and big-endian CPUs with identical kernel command lines.

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