MIPS: Alchemy: devboard register abstraction

All Alchemy development boards have external CPLDs with a few registers
in them.  They all share an identical register layout with only a few
minor differences (except the PB1000) in bit functions and base
addresses.

This patch
- adds a primitive facility to initialize and use these external
  registers,
- replaces all occurrences of bcsr->xxx accesses with calls to the new
  functions (the pb1200 cascade irq handling code is special).
- collects BCSR register information scattered throughout the board
  headers in a central place.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>