ARM: sirf: mark sirfsoc_init_late as __maybe_unused

sirfsoc_init_late is called by each of the three individual
SoC definitions, but in a randconfig build, we can encounter
a situation where they are all disabled:

arch/arm/mach-prima2/common.c:18:123: warning: 'sirfsoc_init_late' defined but not used [-Wunused-function]

While that is not a useful configuration, the warning also
doesn't help, so this patch marks the function as __maybe_unused
to let the compiler know it is there intentionally.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>