ram: stm32mp1: the property st, phy-cal becomes optional

ram: stm32mp1: the property st, phy-cal becomes optional

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.

This patch also impact the ddr interactive mode
- the registers of the param 'phy.cal' are initialized to 0 when
  "st,phy-cal" is not present in device tree (default behavior when
  DDR_PHY_CAL_SKIP is not activated)
- the info 'cal' field can be use to change the calibration behavior
  - cal=1 => use param phy.cal to initialize the PHY, built-in training
             is skipped
  - cal=0 => param phy.cal is absent, built-in training is used (default)

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>