powerpc: Merge i8259.c into arch/powerpc/sysdev

This changes the parameters for i8259_init so that it takes two
parameters: a physical address for generating an interrupt
acknowledge cycle, and an interrupt number offset.  i8259_init
now sets the irq_desc[] for its interrupts; all the callers
were doing this, and that code is gone now.  This also defines
a CONFIG_PPC_I8259 symbol to select i8259.o for inclusion, and
makes the platforms that need it select that symbol.

Signed-off-by: Paul Mackerras <paulus@samba.org>