xtensa: xtfpga: fix i2c controller register width and endianness

I2C controller is attached to XTFPGA boards as native endian device, mark
it as such in DTS.
Set register width in DTS to 4, this way it works both for little- and
big-endian CPUs.

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