regmap: Introduce regmap_get_reg_stride.

This patch introduces regmap_get_reg_stride() function which would
be used by the infrastructures like eeprom 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>