regmap: mmio: Add support for 1/2/8 bytes wide register address.

Since regmap core and mmio have already support for 1/2/8 bytes wide values,
so adds support for 1/2/8 bytes wide registers address.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>