MIPS: pistachio: remove obsolete include/asm/mach-pistachio

Since commit 02bd530f ("MIPS: generic: Increase NR_IRQS to 256")
include/asm/mach-pistachio/irq.h just does nothing.
Remove the file along with mach-pistachio folder and include compiler
directive.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>