stm32mp1: ram: add read valid training support

Add the read data eye training = training for optimal read valid placement
(RVTRN) when the built-in calibration is executed for LPDDR2 and LPDDR3.

This training is supported on the PUBL integrated in the STM32MP15x
DDR subsystem and it is not required for DDR3.

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