mmc: sunxi: Support vqmmc regulator

eMMC chips require 2 power supplies, vmmc for internal logic, and vqmmc
for driving output buffers. vqmmc also controls signaling voltage. Most
boards we've seen use the same regulator for both, nevertheless the 2
have different usages, and should be set separately.

This patch adds support for vqmmc regulator supply, including voltage
switching. The MMC core can use this to try different signaling voltages
for eMMC.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>