Add hooks to regulator_get_current_limit_regmap() and
regulator_set_current_limit_regmap() with an accompanying map of amperages.

This lets us use the existing helper functions to map requested current
settings to register bit-map/indicies.

This change is required to elevate the default 2 Amps set by the bootloader
to 3 Amps or indeed to constrain the value lower as the system design may
dictate.

The valid range is 500 mA to 3 A in increments of 500 mA.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Link: https://lore.kernel.org/r/20210427130712.2005456-2-bryan.odonoghue@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>