Add mfdec emulation

We support setting the DEC to a certain value right now. Doing that basically
triggers the CPU local timer.

But there's also an mfdec command that enabled the OS to read the decrementor.

This is required at least by all desktop and server PowerPC Linux kernels. It
can't really hurt to allow embedded ones to do it as well though.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>