m68k/irq: Rename irq_controller to irq_chip

Make it more similar to the genirq version:
  - Remove lock (unused as we don't do SMP anyway),
  - Prepend methods with irq_,
  - Make irq_startup() return unsigned int.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>