ARM: dts: cygnus: Enable Performance Monitoring Unit

Add PMU capability to Cygnus so trace and performance profiling
can be used.

Signed-off-by: Jason Uy <jason.uy@broadcom.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>