arm64: add EFI runtime services

This patch adds EFI runtime support for arm64. This runtime support allows
the kernel to access various EFI runtime services provided by EFI firmware.
Things like reboot, real time clock, EFI boot variables, and others.

This functionality is supported for little endian kernels only. The UEFI
firmware standard specifies that the firmware be little endian. A future
patch is expected to add support for big endian kernels running with
little endian firmware.

Signed-off-by: Mark Salter <msalter@redhat.com>
[ Remove unnecessary cache/tlb maintenance. ]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>