mmc: sdhci-xenon: Add support to PHYs of Marvell Xenon SDHC

Marvell Xenon eMMC/SD/SDIO Host Controller contains PHY.
Multiple types of PHYs are supported.

Add support to multiple types of PHYs init and configuration.
Add register definitions of PHYs.

Xenon PHY cannot fit in kernel common PHY framework.
Xenon SDHC PHY register is a part of Xenon SDHC register set.
Besides, MMC initialization has to call several PHY functions to
complete timing setting.
Those PHY setting functions have to access SDHC registers and know
current MMC setting, such as bus width, clock frequency and
speed mode.
As a result, implement Xenon PHY in MMC host directory.

Signed-off-by: Hu Ziji <huziji@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>