ARM: uniphier: skip memreserve of unused DRAM bank of LD20

Now the "for" loop here iterates on the detected memory banks.
It must skip unused DRAM banks.

Fixes: c995f3a3 ("ARM: uniphier: use gd->bd->bi_dram for memory reserve on LD20 SoC")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>