dt-bindings: clock: fixed: Define a preferred node name

Define "clock-<freq>" as the preferred node name for fixed-clock and
fixed-factor-clock where <freq> is the output frequency of the clock.
There isn't much of an existing pattern for names of these nodes. The
most frequent patterns are a prefix or suffix of "clk", but there's a
bunch that don't follow any sort of pattern. We could use
"clock-controller-.*", but these nodes aren't really a controller in any
way. So let's at least align with part of that and use 'clock-'.

For now this only serves as documentation as the schema still allows
anything to avoid lots of additional warnings for something low priority
to fix. Once a "no deprecated" mode is added to the tools, warnings can
be enabled selectively.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20240430180415.657067-1-robh@kernel.org
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>