power: Add support for handling the Decrementer Interrupt

power: Add support for handling the Decrementer Interrupt

This patch allows the programming of the decrementer device, which
will count down to zero. As of now, the decrement happens after every
instruction. When the decrementer value hits 0, the CPU is delivered a
decrementer interrupt.

[ego@linux.vnet.ibm.com: Fixed Conflicts in src/arch/power/interrupts.hh]

Change-Id: I3a863a8e2bca434d5a8139df662429d3e83a8542

Signed-off-by: Phanikiran Harithas <phanikiran.harithas@gmail.com>
Signed-off-by:  Venkatnarayan Kulkarni <venkatnarayankulkarni@gmail.com>