MIPS: traps: Add CPU PM callback for trap configuration

Implement a CPU power management callback for restoring trap related CPU
configuration after CPU power up from a low power state. The following
state is restored:

- Status register
- HWREna register
- Exception vector configuration registers
- Context/XContext register

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Paul Burton <paul.burton@imgtec.com>