ARM: realview: fix up PB11MP flash compat strings

The two flash memories in the PB11MPCore have their VPP/WP
lines controlled from the system controller add-on in the MTD
subsystem. "arm,versatile-flash" is the first compatible string
to use to get the right support.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>