iio: dac: mcp4725: use regulator framework

Use a standard framework to get the reference voltage. It is done that way
in the iio subsystem and it will simplify extending of the driver.

Structure mcp4725_platform_data is left undeleted because it used in the
next patch.

This change breaks the current users of the driver, but there is no
mainline user of struct mcp4725_platform_data.

Signed-off-by: Tomas Novotny <tomas@novotny.cz>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>