coresight: cti: Access ASICCTL only when implemented

coresight: cti: Access ASICCTL only when implemented

According to the Arm ARM (DDI 0487 L.b), ASICCTL is implemented only
when CTIDEVID.EXTMUXNUM is non-zero.

Based on CTIDEVID.EXTMUXNUM, add a flag 'asicctl_impl' to indicate
whether the register is implemented, and access ASICCTL conditionally
based on the flag.

Allow the sysfs node to be visible only when the register is
implemented.

Reviewed-by: Mike Leach <mike.leach@arm.com>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20260226-arm_coresight_cti_refactor_v1-v2-3-b30fada3cfec@arm.com