MIPS: VR41xx: Convert to new irq_chip functions

And cleanup direct access to irq_desc[].

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2208/
Patchwork: https://patchwork.linux-mips.org/patch/2209/


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