Add dummy implementation of generic timer cp15 registers

Add dummy implementation of generic timer cp15 registers

Add a dummy implementation of the cp15 registers for the generic
timer (found in the Cortex-A15), just sufficient for Linux to
decide that it can't use it. This requires at least CNTP_CTL and
CNTFRQ to be implemented as RAZ/WI; we RAZ/WI all of c14.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>