MIPS: BMIPS: Let each platform customize the CPU1 IRQ mask

On some chips like bcm3384, "other stuff" gets wired up to CPU1's IE_IRQ1
input, generating spurious IRQs.  In this case we want the platform code
to be able to mask it off.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: f.fainelli@gmail.com
Cc: mbizon@freebox.fr
Cc: jogo@openwrt.org
Cc: jfraser@broadcom.com
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8163/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>