mfd: madera: Add supply mapping for MICVDD

Currently we are relying on the exact match of the regulator name to
find MICVDD, we should add an explicit supply mapping to allow this to
be found more reliably.

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