ssb: pick SoC invariants code from MIPS BCM47xx arch

There is code in ssb fetching "invariants" that is basically a set of
board specific data. Every host requires its own implementation of
reading function. In ssb we have support for PCI, PCMCIA & SDIO.
For some (historical?) reason code reading "invariants" for SoC was
placed in arch code and provided by a callback. This is not needed
nowadays, so lets move that into ssb. This way we keep all "invariants"
functions in a single module making code cleaner.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>