powerpc: move the install rule to arch/powerpc/Makefile

Currently, the install target in arch/powerpc/Makefile descends into
arch/powerpc/boot/Makefile to invoke the shell script, but there is no
good reason to do so.

arch/powerpc/Makefile can run the shell script directly.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210729141937.445051-3-masahiroy@kernel.org