arm64: Add BOOT_TARGETS variable

Add a new variable containing a list of possible targets. Mark them as
phony. This matches the approach taken for arch/arm

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Link: https://lore.kernel.org/r/20240329032836.141899-2-sjg@chromium.org


Signed-off-by: Will Deacon <will@kernel.org>