MIPS: PM: Add CPU PM callbacks for general CPU context

Add a CPU power management notifier callback for preserving general CPU
context. The CPU PM callbacks will be triggered by the powering down of
CPU cores, for example by cpuidle drivers & in the future by suspend to
RAM implementations.

The current state preserved is mostly related to the process context:
- FPU
- DSP
- ASID
- UserLocal
- Watch registers

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 5cd695f..322bbe1 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -50,6 +50,7 @@
 	select CLONE_BACKWARDS
 	select HAVE_DEBUG_STACKOVERFLOW
 	select HAVE_CC_STACKPROTECTOR
+	select CPU_PM if CPU_IDLE
 
 menu "Machine selection"