dt-bindings: mmc: mmci: add delay block base register for sdmmc

To support the sdr104 mode, the sdmmc variant has a hardware delay block to
manage the clock phase when sampling data received by the card.

This patch adds a second base register (optional) for sdmmc delay block.
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200128090636.13689-6-ludovic.barre@st.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mmci.txt

@@ -28,6 +28,8 @@ specific for ux500 variant:
 - st,sig-pin-fbclk       : feedback clock signal pin used.
 
 specific for sdmmc variant:
+- reg			 : a second base register may be defined if a delay
+                           block is present and used for tuning.
 - st,sig-dir             : signal direction polarity used for cmd, dat0 dat123.
 - st,neg-edge            : data & command phase relation, generated on
                            sd clock falling edge.