Revert "fdt_support: Use CONFIG_NR_DRAM_BANKS if defined"

This reverts commit 5e574546.

The reverted commit didn't support the scenario where there are less
DRAM banks in U-Boot than in Linux.
Also, it didn't introduce any new functionality, only limitaion.
User could just increase MEMORY_BANKS_MAX if it's too small.
Reviewed-by: Simon Glass <sjg@chromium.org>