ARM: rmobile: Switch back to fdtdec_setup_memory/banksize_fdt()

The commit 361377db ("ARM: rmobile: Merge prior-stage firmware DT
fragment into U-Boot DT on Gen3") reverted changes introduced by commit
175f5027 ("ARM: renesas: Configure DRAM size from ATF DT fragment")
that's why there is no reason to use functions with _fdt() suffix because
parameter is gd->fdt_blob as is already for functions without _fdt()
suffix.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>