ARM: rmobile: Disable CONFIG_ARCH_FIXUP_FDT_MEMORY

Disable CONFIG_ARCH_FIXUP_FDT_MEMORY to prevent U-Boot from modifying
the memory {} nodes in the DT passed to the Linux kernel. The R8A779x
DT contains multiple memory {} nodes, while U-Boot only modifies the
first one and stuffs all the memory entries into it, which is wrong.
Disabling CONFIG_ARCH_FIXUP_FDT_MEMORY is the least intrusive way to
fix the issue this close to the release, while the real fix is to
extend the fdt_fixup_memory_banks() to handle multiple memory nodes
in DT.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>