ARM: rename CONFIG_TIMER_CLK_FREQ to COUNTER_FREQUENCY

ARM: rename CONFIG_TIMER_CLK_FREQ to COUNTER_FREQUENCY

Many ARMv8 boards define a constant COUNTER_FREQUENCY to specify the
frequency of the ARM Generic Timer (aka. arch timer).
ARMv7 boards traditionally used CONFIG_TIMER_CLK_FREQ for the same
purpose. It seems useful to unify them.
Since there are less occurences of the latter version, lets convert all
users over to COUNTER_FREQUENCY.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>