Add support for non-zero irq base to mv64360_pic code.

- Fix mv64360 pic code to handle non-zero mv64x60_irq_base
- Cleanup mv64360 entries in /proc/interrupts

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Mark A. Greer <mgreer@mvista.com>