ARM: dts: Fix up the D-Link DIR-685 MTD partition info

The vendor firmware was analyzed to get the right idea about
this flash layout. /proc/mtd contains:

dev:    size   erasesize  name
mtd0: 01e7ff40 00020000 "rootfs"
mtd1: 01f40000 00020000 "upgrade"
mtd2: 00040000 00020000 "rgdb"
mtd3: 00020000 00020000 "nvram"
mtd4: 00040000 00020000 "RedBoot"
mtd5: 00020000 00020000 "LangPack"
mtd6: 02000000 00020000 "flash"

Here "flash" is obviously the whole device and we know "rootfs"
is a bogus hack to point to a squashfs rootfs inside of the main
"upgrade partition". We know "RedBoot" is the first 0x40000 of
the flash and the "upgrade" partition follows from 0x40000 to
0x1f8000. So we have mtd0, 1, 4 and 6 covered.

Remains:
mtd2: 00040000 00020000 "rgdb"
mtd3: 00020000 00020000 "nvram"
mtd5: 00020000 00020000 "LangPack"

Inspecting the flash at 0x1f8000 and 0x1fa000 reveals each of
these starting with "RGCFG1" so we assume 0x1f8000-1fbfff is
"rgdb" of 0x40000.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>