coresight: Refactor out buffer allocation function for ETR

When CATU is moved to the generic enable/disable path system in the
next commit, it will need to call into ETR and get it to pre-allocate
its buffer so add a function for it.

No functional changes

Reviewed-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: James Clark <james.clark@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20230425143542.2305069-12-james.clark@arm.com