OF: MIPS: lantiq: implement irq_domain support

OF: MIPS: lantiq: implement irq_domain support

Add support for irq_domain on lantiq socs. The conversion is straight forward
as the ICU found inside the socs allows the usage of irq_domain_add_linear.

Harware IRQ 0->7 are the generic MIPS IRQs. 8->199 are the Lantiq IRQ Modules.
Our irq_chip callbacks need to substract 8 (MIPS_CPU_IRQ_CASCADE) from d->hwirq
to find out the correct offset into the Interrupt Modules register range.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3802/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>