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: NHu Ziji <huziji@marvell.com>
Tested-by: NRussell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>