dt-bindings: clock: silabs,si5341: Add missing properties

dt-bindings: clock: silabs,si5341: Add missing properties

Add "clock-output-names" which is a standard property for clock
providers.

Add the "always-on" boolean property which was undocumented, but
already in use for some time. The flag prevents a clock output from
being disabled.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Tested-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>