regmap: Make reg_config reg_defaults const

The reg_defaults field usually points to a static per driver array, which should
not be modified. Make requirement this explicit by making reg_defaults const.
To allow this the regcache_init code needs some minor changes. Previoulsy the
reg_config was not available in regcache_init and regmap->reg_defaults was used
to pass the default register set to regcache_init. Now that the reg_config is
available we can work on it directly.

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