perf cs-etm: Save TRCDEVARCH register

When ETE is present save the TRCDEVARCH register and set a new magic
number. It will be used to configure the decoder in a later commit.

Old versions of perf will not be able to open files with this new magic
number, but old files will still work with newer versions of perf.

Signed-off-by: James Clark <james.clark@arm.com>