arm64: kernel: re-export _cpu_resume() from sleep.S

Commit b5fe2429 ("arm64: kernel: fix style issues in sleep.S")
changed the linkage of _cpu_resume() to local, even though the symbol
is also referenced from hibernate.c. So revert this change.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>