dt-bindings: Add en7523-scu device tree binding documentation

Adds device tree binding documentation for clocks in the EN7523 SOC.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20220314084409.84394-2-nbd@nbd.name


Signed-off-by: Stephen Boyd <sboyd@kernel.org>