Only register as the platform DELAY() implementation if the setup of the

Only register as the platform DELAY() implementation if the setup of the
global timer was successful, since the implementation tries to read it.

Notably, if the platform has a variable-frequency global timer (because
of dynamic frequency scaling), it doesn't set up the global timer for use
as a system timecounter, and in that case it also can't use it for DELAY.
Such platforms use different timer hardware for both timecounter and DELAY.