pcmcia: soc_common: add support for Vcc and Vpp regulators

Add support for handling supply regulators in the soc_common code.  This
allows us to separate out the board specifics for setting voltages from
the PCMCIA code.

We detect when setting a voltage fails, and report this fact - some
platforms have fixed-voltage supplies (eg, for CF sockets at 3.3V) and
we need to ignore attempts to configure for 5V, as per the existing
board specific drivers.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>