mmc: xenon: set signal voltage and max base clock

mmc: xenon: set signal voltage and max base clock

- The SDIO signal voltage and max base clock frequency
  setting are missing in driver, which causes SDIO
  not working.
- The patch adds SDIO signal voltage switch support,
  which is based on regulator-gpio of vqmmc-supply, and
  sets the max base clock frequency.
- Fix the zero clock value in call to sdhci_setup_cfg()
  function.

Change-Id: I79c8860c65b8db166f4f70db56ede4097f71f1fa
Signed-off-by: Evan Wang <xswang@marvell.com>
Reviewed-on: http://vgitil04.il.marvell.com:8080/53589
Reviewed-by: Hua Jing <jinghua@marvell.com>
Tested-by: Hua Jing <jinghua@marvell.com>
[pali: Amended fixup patch]
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Konstantin Porotchkin <kostap@marvell.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Andre Heider <a.heider@gmail.com>