coresight: etm4x: Hide sysfs attributes for unavailable registers

coresight: etm4x: Hide sysfs attributes for unavailable registers

Some of the management registers in ETMv4.x are not accessible
via system register instructions. Thus we must hide the sysfs
files exposing them to the userspace, to prevent system crashes.

This patch adds an is_visible() routine to control the visibility
at runtime for the registers that may not be accessed.

Link: https://lore.kernel.org/r/20210110224850.1880240-13-suzuki.poulose@arm.com
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210201181351.1475223-15-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>