ARM: rmobile: salvator-x: Count all DRAM in all slots

Instead of counting only the DRAM in the first slot, count
all the DRAM in all slots and report it accordingly.

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