Makefile: Change COUNTER_FREQ to 100 MHz

Older Arm Fast Models (AEM < RevC) had a base frequency of 24 MHz. but
the RevC base models use 100 MHz. There is not a robust method of
determining the configured base frequency at runtime, so update
COUNTER_FREQ to be 100 MHz.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>