dt-bindings: clock: st: flexgen: remove deprecated compatibles

st/stih407-clock.dtsi file has been removed in commit 65322c1daf51
("clk: st: flexgen: remove unused compatible").  This file has three
compatibles which are now dangling.  Remove them from documentation.

Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Brian Masney <bmasney@redhat.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>