MIPS: BCM63xx: Protect irq register accesses

MIPS: BCM63xx: Protect irq register accesses

Since we will have the chance of accessing the registers concurrently,
protect any accesses through a spinlock.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: John Crispin <blogic@openwrt.org>
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Gregory Fong <gregory.0xf0@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/7321/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>