MIPS: Alchemy: devboards: factor out PB1200 IRQ cascade code.

Move the PB1200 IRQ cascade code out to the BCSR support code:
upcoming DB1300 support can use it too.

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