microblaze_v8: Interrupt handling and timer support

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@elte.hu>
Reviewed-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Acked-by: John Linn <john.linn@xilinx.com>
Acked-by: John Williams <john.williams@petalogix.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>