rcar_gen3: plat: Print DRAM bank size in MiB if below 1 GiB

Print the DRAM bank size in MiB instead of GiB in case the bank size
is smaller than 1 GiB. This prevents printing zeroes on systems with
small DRAM sizes.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>