dt-bindings: clock: drop obsolete stericsson,abx500.txt

These bindings are already (better) described in mfd/stericsson,ab8500.yaml,
drop these now obsolete bindings.
Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/Zm7I2Zbq1JNPoEJp@standask-GA-A55M-S2HPSigned-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/clock/stericsson,abx500.txt

-Clock bindings for ST-Ericsson ABx500 clocks
-
-Required properties :
-- compatible : shall contain the following:
-  "stericsson,ab8500-clk"
-- #clock-cells should be <1>
-
-The ABx500 clocks need to be placed as a subnode of an AB8500
-device node, see mfd/ab8500.txt
-
-All available clocks are defined as preprocessor macros in
-dt-bindings/clock/ste-ab8500.h header and can be used in device
-tree sources.
-
-Example:
-
-clock-controller {
-	compatible = "stericsson,ab8500-clk";
-	#clock-cells = <1>;
-};