ARM: dts: arm: realview: Fix development chip ROM compatible value

When the development chip ROM was added, the "direct-mapped" compatible
value was already obsolete.  In addition, the device node lacked the
accompanying "probe-type" property, causing the old physmap_of_core
driver to fall back to trying all available probe types.
Unfortunately this fallback was lost when the DT and pdata cases were
merged.

Fix this by using the modern "mtd-rom" compatible value instead.

Fixes: 5c3f5edb ("ARM: realview: add flash devices to the PB1176 DTS")
Fixes: 642b1e8d ("mtd: maps: Merge physmap_of.c into physmap-core.c")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>