dt-bindings: vendor-prefixes: Add 'fsi[ab]' to the list of non-vendor prefixes

dt-bindings: vendor-prefixes: Add 'fsi[ab]' to the list of non-vendor prefixes

'fsia' and 'fsib' property prefixes were added as schema in commit
e56bd85d26ee ("ASoC: fsi: switch to yaml base Documentation").
Unfortunately to do checks on actual vendor prefixes, we have to track
the handful of prefixes which are not vendors like 'fsia' and 'fsib'.

Fixes: e56bd85d26ee ASoC: fsi: switch to yaml base Documentation
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191108153538.11970-1-robh@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>