MFC r271595, r271601, r271607, r271630:

MFC r271595, r271601, r271607, r271630:

  Add compat strings for all the flavors of GIC this driver should support.
  Also allow the driver to attach to ofwbus as well as simplebus, some FDT
  data puts the root interrupt controller on the root bus.

  Add a common routine for parsing FDT data describing an ARM GIC interrupt.

  Use gic_decode_fdt() rather than a local routine to parse fdt interrupt
  properties.  Move fdt_pic_table and fdt_fixup_table into imx6_machdep.c,
  which means imx6 doesn't need imx_common.c anymore.

  The private peripheral interrupts start at offset 16, not 0.  Also, use
  names rather than inline mystery constants for these offsets.