mfd: arizona: Map MICVDD from extcon device to the Arizona core

The extcon driver should be able to get its regulator against rather
than against the main arizona device, we must add a supply mapping
allowing the regulator to be located. This patch does so.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>