regulator: Update regulator_register() API signature in Documentation

commit c172708d "regulator: core: Use a struct to pass in regulator runtime
configuration" changed the regulator_register() API signature.

Update the Documentation accordingly to reflect the change in the function
signature.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>