mmc: sdhci-pxav3: Extend binding with SDIO3 conf reg for the Armada 38x

The SDHCI unit used on the Armada 38x needs using an extra register to
do specific clock adjustments in order to support the SDR50 and DDR50
modes. This patch extends the binding to allow using this register.

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