arm64/kexec: Add core kexec support

arm64/kexec: Add core kexec support

Add three new files, kexec.h, machine_kexec.c and relocate_kernel.S to the
arm64 architecture that add support for the kexec re-boot mechanism
(CONFIG_KEXEC) on arm64 platforms.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Reviewed-by: James Morse <james.morse@arm.com>
[catalin.marinas@arm.com: removed dead code following James Morse's comments]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Conflicts:
	arch/arm64/Kconfig
	arch/arm64/kernel/Makefile
Makefile was wrongly modified when merging:
commit 762f201f0a44 ("arm64: kernel: implement ACPI parking protocol")