[SANTINO] Add support to Santino board

The board has been ported starting from the NXP's iMX6 Sabresd project backbone. Then Seco's iMX6 A62 and A75 have been used as reference to maintain coding uniformity.

Something to highlight is the PMIC handling. NE colleagues said that it may happen that a PMIC mounted on the board may have the DDR voltage set pre-programmed to 1.5V instead of the required 1.35V.

The FnG system configures the PMIC before initializing the DDR. It has been tested in U-Boot in the same condition trying to configure the PMIC in the SPL before intializing the DDR but using the driver's API lead to memory allocation error. For this reason, the PMIC configuration is right-after DDR initialization.

JIRA Reference: https://secogroup.atlassian.net/browse/SANTINO-1