board: verdin-am62: fix check for minimum memory size

verdin am62 SKUs comes in multiple memory configuration, check that
the detected memory is at least 512MB since we have some
reserved memory just before this threshold and therefore
the module cannot work with less memory.

Fixes: 7d1a1065 ("board: toradex: add verdin am62 support")
Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Acked-by: Francesco Dolcini <francesco.dolcini@toradex.com>