ARM: realview: enable multiplatform

All obstacles are out of the way by now, so we can finally move realview
to multiplatform.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>
[Rebased Kconfig, fixed if $(X) to if X in Makefile]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>