arm64: Remove unneeded extern keyword

Function prototypes are never definitions, so remove any 'extern' keyword
from the funcion prototypes in cpu_ops.h. Fixes warnings emited by
checkpatch.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>