regmap: Add endianness support

Add support for switching the endianness of regmap accesses via the
"little-endian", "big-endian", and "native-endian" boolean properties in
the device tree.

The default endianness is native endianness.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>