coresight: etm4x: Make offset available for sysfs attributes

coresight: etm4x: Make offset available for sysfs attributes

Some of the ETM management registers are not accessible via
system instructions. Thus we need to filter accesses to these
registers depending on the access mechanism for the ETM at runtime.
The driver can cope with this for normal operation, by regular
checks. But the driver also exposes them via sysfs, which now
needs to be removed.

So far, we have used the generic coresight sysfs helper macros
to export a given device register, defining a "show" operation
per register. This is not helpful to filter the files at runtime,
based on the access.

In order to do this dynamically, we need to filter the attributes
by offsets and hard coded "show" functions doesn't make this easy.
Thus, switch to extended attributes, storing the offset in the scratch
space. This allows us to implement filtering based on the offset and
also saves us some text size. This will be later used for determining
a given attribute must be "visible" via sysfs.

Link: https://lore.kernel.org/r/20210110224850.1880240-10-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/20210201181351.1475223-12-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>