armv7R: K3: j721e: Add support for triggering ddr init from SPL

In SPL, DDR should be made available by the end of board_init_f()
so that apis in board_init_r() can use ddr. Adding support for
triggering DDR initialization from board_init_f().

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>