ARM: realview: remove private barrier implementation

The realview barrier implementation tries to avoid calling outer_sync in order
to not lock up as a result of a bug in the l220 cache controller.

This gets in the way of the multiplatform support, but we can still remove
it if we make sure that the outer_sync function never gets called, by replacing
the function pointer with NULL, right after initialization.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[Fixed up header inclusions]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>