scripts: get_feat.pl: improve matrix output

Instead of producing a too wide table, let's split it per
subsystem, and use a better notation in order to make easier
for the reader to identify how a feature is supported on
multiple architectures.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9c3e8c813e8146c5c30e10fa75974f8fbfe6016a.1606748711.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>