Add support for CONFIG_REGULATOR_FIXED_VOLTAGE.

Without this option the mx27_3ds cannot have the external Ethernet functional
due to the need of smsc regulators.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>