This parameter "st,phy-cal" becomes optional and when it is
absent the built-in PHY calibration is done.

It is the case in the helper dtsi file "stm32mp15-ddr.dtsi"
except if DDR_PHY_CAL_SKIP is defined.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: I38ffd4aeca88fd45819d4e7d20840465a8a8e9bf
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/151296


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>