MIPS/IRQCHIP: Move irq_chip from arch/mips to drivers/irqchip.

While at it, rename it because in drivers/irqchip no longer every CPU is
a MIPS.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/txx9/Kconfig

@@ -8,7 +8,7 @@ config MACH_TX49XX
 	select MACH_TXX9
 	select CEVT_R4K
 	select CSRC_R4K
-	select IRQ_CPU
+	select IRQ_MIPS_CPU
 	select SYS_HAS_CPU_TX49XX
 	select SYS_SUPPORTS_64BIT_KERNEL