armv7R: K3: am654: 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().

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>