regulator: core: Add regulator_is_equal() helper

Add regulator_is_equal() helper to compare whether two regulators are
the same. This is useful for checking whether two separate regulators
in a driver are actually the same supply.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Igor Opaniuk <igor.opaniuk@toradex.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Link: https://lore.kernel.org/r/20191220164450.1395038-1-marex@denx.de


Signed-off-by: Mark Brown <broonie@kernel.org>