[POWERPC] Export virq mapping via debugfs

This adds a debugfs file "powerpc/virq_mapping", which shows the virtual
to real mapping of irq numbers.  Enable it with CONFIG_VIRQ_DEBUG.

Signed-off-by: Zhang Wei <wei.zhang@freescale.com>
Signed-off-by: Chen Gong <G.Chen@freescale.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>