[POWERPC] Copy i8259 code back to arch/ppc

This copies the i8259 interrupt controller driver from arch/powerpc
to arch/ppc. It's currently shared by both architectures, but the upcoming
arch/powerpc interrupt changes will break the arch/ppc builds. The changes
are too important to just use #ifdef's in the driver.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>