regmap: Introduce regmap_get_reg_stride

This patch introduces regmap_get_reg_stride() function which would
be used by the infrastructures like nvmem framework built on top of
regmap. Mostly this function would be used for sanity checks on inputs
within such infrastructure.

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