dt-bindings: clock: adi,axi-clkgen: convert old binding to yaml format

dt-bindings: clock: adi,axi-clkgen: convert old binding to yaml format

This change converts the old binding for the AXI clkgen driver to a yaml
format.

As maintainers, added:
 - Lars-Peter Clausen <lars@metafoo.de> - as original author of driver &
   binding
 - Michael Hennerich <michael.hennerich@analog.com> - as supporter of
   Analog Devices drivers

Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201013143421.84188-1-alexandru.ardelean@analog.com
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>