mmc: sdhci-xenon: Add SoC PHY PAD voltage control

Some SoCs have PHY PAD outside Xenon IP.
PHY PAD voltage should match signalling voltage in use.

Add generic SoC PHY PAD voltage control interface.
Implement Aramda-3700 SoC PHY PAD voltage control.

Signed-off-by: Hu Ziji <huziji@marvell.com>
Tested-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>