regmap: Add explict native endian flag to DT bindings

Currently the binding document says that if no endianness is configured
we use native endian but this is not in fact true for all binding types
and we do have some devices that really want native endianness such as
Broadcom MIPS SoCs where switching the endianness of the CPU also
switches the endianness of external IPs.

Provide an explicit option for this.

Signed-off-by: Mark Brown <broonie@kernel.org>