powerpc: Add accounting for Doorbell interrupts

This patch adds a new line to /proc/interrupts to account for the
doorbell interrupts that each hardware thread has received. The total
interrupt count in /proc/stat will now also include doorbells.

 # cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3
 16:        551       1267        281        175      XICS Level     IPI
LOC:       2037       1503       1688       1625   Local timer interrupts
SPU:          0          0          0          0   Spurious interrupts
CNT:          0          0          0          0   Performance monitoring interrupts
MCE:          0          0          0          0   Machine check exceptions
DBL:         42        550         20         91   Doorbell interrupts

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>