coresight: etm4x: Add sysreg access helpers

ETM architecture defines the system instructions for accessing
via register accesses. Add basic support for accessing a given
register via system instructions.

We split the list of registers as :
 1) Accessible only from memory mapped interface
 2) Accessible from system register instructions.

All registers are accessible via the memory-mapped interface.
However, some registers are not accessible via the system
instructions. This list is then used to further filter out
the files we expose via sysfs.

Link: https://lore.kernel.org/r/[email protected]
Cc: Mike Leach <[email protected]>
Cc: Mathieu Poirier <[email protected]>
Signed-off-by: Suzuki K Poulose <[email protected]>
Signed-off-by: Mathieu Poirier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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