libfdt: replace ARCH_FIXUP_FDT with ARCH_FIXUP_FDT_MEMORY

Commit e2f88dfd ("libfdt: Introduce new ARCH_FIXUP_FDT option")
allows us to skip memory setup of DTB, but a problem for ARM is that
spin_table_update_dt() and psci_update_dt() are skipped as well if
CONFIG_ARCH_FIXUP_FDT is disabled.

This commit allows us to skip only fdt_fixup_memory_banks() instead
of the whole of arch_fixup_fdt().  It will be useful when we want to
use a memory node from a kernel DTB as is, but need some fixups for
Spin-Table/PSCI.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Simon Glass <sjg@chromium.org>
Fixed build error for x86:
Signed-off-by: Simon Glass <sjg@chromium.org>