ARM: fix missing CLKSRC_OF on multi-platform

In commit ddb902cc

 (ARM: centralize common multi-platform kconfig
options), CLKSRC_OF was removed from some platforms, but not added to
ARCH_MULTIPLATFORM. Fix this.

Reported-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>