arm: mach-k3: j721s2_init: Support less than max DDR controllers

The number of DDR controllers to be initialised and used should depend
on the device tree with the constraint of the maximum number of
controllers the device supports. Since J721S2 has multiple (2)
controllers, instead of hardcoding the number of probes, move to
depending on the device tree UCLASS_RAM nodes present.

Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Manorit Chawdhry <m-chawdhry@ti.com>