ARM: imx8mp: verdin-imx8mp: Add memory size detection

Add RAM auto-sizing, without this change memory size for all SKU is set
to 8GB and the system will crash on SKU with less memory as soon as the
non existent memory addresses are accessed.

Fixes: 2bc2f817 ("board: toradex: add verdin imx8m plus support")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>