arm: mach-bcm283x: Add ARMV8_MULTIENTRY support

arm: mach-bcm283x: Add ARMV8_MULTIENTRY support

When ACPI is enabled over FDT the APs cannot be brought out of reset
by the OS using the "FDT spin-table" mechanism, as no FDT is provided
to the OS. The APs must be released out of reset in u-boot and then
brought up in an ACPI compliant fashion.

When ARMV8_MULTIENTRY is specified, the APs are released from reset
and will enter U-Boot after it has been relocated as well.

By default ARMV8_MULTIENTRY is not selected, keeping existing behaviour.

TEST: All APs enter U-Boot when run on qemu-system-aarch64 and on
      real hardware.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Matthias Brugger <mbrugger@suse.com>
Cc: Peter Robinson <pbrobinson@gmail.com>
Cc: Tom Rini <trini@konsulko.com>