powerpc/64s: ppc_save_regs is now needed for all 64s builds

Commit 78adf6c2 ("powerpc/64s: Implement system reset idle wakeup
reason"), added a call to ppc_save_regs() in the book3s code.

ppc_save_regs() is only built if XMON and/or KEXEC_CORE are enabled,
which is usually the case, however if they're not enabled then the
build breaks.

Fix it by making the Makefile check also build ppc_save_regs.o if
CONFIG_PPC_BOOK3S is enabled.

Fixes: 78adf6c2

 ("powerpc/64s: Implement system reset idle wakeup reason")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
[mpe: Write change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>