regmap: mmio: Parse endianness definitions from DT

Since we changed to do formatting in the bus we now skip all the format
parsing that the core does for its data marshalling code.  This means
that we skip the DT parsing it does which breaks some systems, we need
to add an explict call in the MMIO code to do this.

Reported-by: Alexander Stein <alexander.stein@systec-electronic.com>
Tested-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Mark Brown <broonie@kernel.org>