MIPS: Loongson64: Implement PM suspend for LEFI firmware

Implement PM suspend for LEFI firmware.
Entering STR (Suspend to RAM) is as simple as save our context
then go to a firmware vector.
Wake is a little bit treaky as we need to setup some CP0 status
first, which can be done with smp_slave_setup.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>