regmap: Introduce regmap_get_max_register

This patch introduces regmap_get_max_register() function which would be
used by the infrastructures like nvmem framework built on top of
regmap.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>