lib: fdt: Split fdtdec_setup_memory_banksize()

Split fdtdec_setup_memory_banksize() into fdtdec_setup_memory_banksize_fdt(),
which allows the caller to pass custom blob into the function and the
original fdtdec_setup_memory_banksize(), which uses the gd->fdt_blob. This
is useful when configuring the DRAM properties from a FDT blob fragment
passed in by the firmware.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>