regmap: Use pad_bits and reg_bits when determining register format.

This change combines any padding bits into the register address bits when
determining register format handlers to use the next byte-divisible
register size.
A reg_shift member is introduced to the regmap struct to enable fixup
of the reg format.
Format handlers now take an extra parameter specifying the number of
bits to shift the value by.

Signed-off-by: Marc Reilly <marc@cpdesign.com.au>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>