arm64: pi: use 'targets' instead of extra-y in Makefile

%.pi.o files are built as prerequisites of other objects.
There is no need to use extra-y, which is planned for deprecation.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20250602180937.528459-1-masahiroy@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>