regulator: Fix regulator_register() API signature in Documentation

The commit 2c043bcb ("regulator: pass additional of_node to
regulator_register()") added an additional parameter to the
regulator_register() API.
Update the Documentation accordingly to reflect the change
in the function signature.

Reported-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>