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 <[e-mail]>
Reviewed-by: Brian Masney <[e-mail]>
Reviewed-by: Patrice Chotard <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>

Documentation/devicetree/bindings/clock/st/st,flexgen.txt

···
   audio use case)
   "st,flexgen-video", "st,flexgen" (enable clock propagation on parent
 					and activate synchronous mode)
-  "st,flexgen-stih407-a0"
   "st,flexgen-stih410-a0"
-  "st,flexgen-stih407-c0"
   "st,flexgen-stih410-c0"
   "st,flexgen-stih418-c0"
-  "st,flexgen-stih407-d0"
   "st,flexgen-stih410-d0"
   "st,flexgen-stih407-d2"
   "st,flexgen-stih418-d2"