regmap: Add helper function for checking if a register range is volatile

We already have the same code for checking whether a register range is volatile
in two different places. Instead of duplicating it once more  add a small helper
function for checking whether a register range is voltaile.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>