It is now possible to build a modular kernel for vexpress by
not setting CONFIG_ARCH_VEXPRESS=y and instead setting =m on the
drivers that it normally implies. This is with the exception of
CLK_VEXPRESS_OSC which is currently hidden behind a dependency on
one of several ARCH_* variables. Remove that dependency so that
CLK_VEXPRESS_OSC may be enabled without it.