regulator: add missing defintion regulator_is_supported_voltage

This definition is missing when CONFIG_REGULATOR is not defined.
This causes compiler errors when compiling sdhci.c.  This can
be worked around by adding #ifdef CONFIG_REGULATOR .. #endif
but since other definitions are there we have defined the missing
definition

Signed-off-by: Philip Rakity <prakity@marvell.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>