mmc: mmci: add variant property to define irq pio mask

This patch allows to define specific pio mask for variants.
Needed to support the STM32 sdmmc variant which has some bits
with different meaning (bits: 21,20,13,12,9)

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>