mfd: ab8500-core: Add AB8500 external regulators as MFD devices

The AB8500 external regulator driver is now a device in its own right,
so it requires registering and probing in the normal way. This patch
will ensure the driver is probed once registered for all devices which
support them.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>