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 <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>