arm64: Fix definition of arm_pm_restart to match the declaration

Commit ff70130 (arm64: use common reboot infrastructure) converted the
arm_pm_restart declaration to the new reboot infrastructure but missed
the actual definition.

Change-Id: I6a5d0d3e4806b53272e7d0c52db933f838f3d487
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: b0946fc84628b8d60e7a2034b48d1aff7da9d1df
Git-Repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/


Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>