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-S2HP


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>