arm64: kbuild: make "make install" not depend on vmlinux

For the same reason as commit 19514fc6 ("arm, kbuild: make "make
install" not depend on vmlinux"), the install targets should never
trigger the rebuild of the kernel.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>