Improve FRV's use of generic IRQ handling:

 (*) Use generic_handle_irq() rather than __do_IRQ() as the latter is obsolete.

 (*) Don't implement enable() and disable() ops as these will fall back to
     using unmask() and mask().

 (*) Provide mask_ack() functions to avoid a call each to mask() and ack().

 (*) Make the cascade handlers always return IRQ_HANDLED.

 (*) Implement the mask() and unmask() functions in the same order as they're
     listed in the ops table.
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>