mmc: sdhci-st: Add macros for register offsets and bitfields for mmcss glue regs

The stih407 family SoC's have additional glue registers in the flashSS which
are used to configure the Arasan controller.

This patch adds macros for the register offsets and bitfields which will be
used by subsequent patches to support stih407 family SoC's.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>