regmap-mmio: Use native endianness for read/write

The regmap API has an endianness setting for formatting reads and writes.
This can be set by the usual DT "little-endian" and "big-endian" properties.
To work properly the associated regmap_bus needs to read/write in native
endian.

The "syscon" DT device binding creates an mmio-based regmap_bus which
performs all reads/writes as little-endian. These values are then converted
again by regmap, which means that all of the MIPS BCM boards (which are
big-endian) have been declared as "little-endian" to get regmap to convert
them back to big-endian.

Modify regmap-mmio to use the native-endian functions __raw_read*() and
__raw_write*() instead of the little-endian functions read*() and
write*().

Modify the big-endian MIPS BCM boards to use what will now be the correct
endianness instead of pretending that the devices are little-endian.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Mark Brown <broonie@kernel.org>