pwm: imx-pwm: add explicit compatible strings and required clock properties

pwm: imx-pwm: add explicit compatible strings and required clock properties

The imx-pwm binding contains language indicating compatible strings to
be used that is not valid for all supported parts e.g. Should be
"fsl,<soc>-pwm". Fix this by enumerating the valid compatible strings
that represent the two versions of this peripheral in use.

The binding is also missing the clocks/clock-names properties so
document these, the two required ipg and per clocks, and add these
properties to the example.

Signed-off-by: Matt Porter <mporter@konsulko.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>