soc bcm: brcmstb: Add support for S2/S3/S5 suspend states (MIPS)

This commit adds support for the Broadcom STB S2/S3/S5 suspend
states on MIPS based SoCs.

This requires quite a lot of code in order to deal with the
different HW blocks that need to be quiesced during suspend:

- DDR PHY
- DDR memory controller and arbiter
- control processor

The final steps of the suspend execute in cache and there is is a little
bit of assembly code in order to shut down the DDR PHY PLL and then go
into a wait loop until a wake-up even occurs. Conversely the resume part
involves waiting for the DDR PHY PLL to come back up and resume
executions where we left.

Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>