MIPS: pm-cps: Generate idle state entry code when CPUs are onlined

The MIPS Coherent Processing System (CPS) power management code has
previously generated code used to enter low power idle states once
during boot for all CPUs. This has the drawback that if a CPU is present
in the system but not being used (for example due to the maxcpus kernel
parameter) then we encounter problems due to not having probed that CPU
for information about its type & properties. The result of this is that
we generate entry code which is both unused, potentially entirely
invalid & likely to be unsuitable for the CPU in question anyway.

Avoid this by generating idle state entry code only when a CPU is
brought online. This way we only ever generate code for CPUs that we
know we've probed the properties of, and that will actually be used.

[ralf@linux-mips.org: Resolve merge conflict.]

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14259/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>