mmc: sdhci-pxav3: Modify clock settings for the SDR50 and DDR50 modes

According to erratum 'FE-2946959' both SDR50 and DDR50 modes require
specific clock adjustments in SDIO3 Configuration register.

This commit add the support of this register and for SDR50 or DDR50
mode use it as suggested by the erratum:
- Set the SDIO3 Clock Inv field in SDIO3 Configuration register to not
inverted.
- Set the Sample FeedBack Clock field to 0x1

[gregory.clement@free-electrons.com: port from 3.10]

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