board: ti: am64x: Account for DDR size fixups if ECC is enabled

Call into k3-ddrss driver to fixup device tree and resize the available
amount of DDR if ECC is enabled.

A second fixup is required from A53 SPL to take the fixup as done from
R5 SPL and apply it to DT passed to A53 U-boot, which in turn passes
this to the OS.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>