doc: dts/macros.rst: "clocks" macro fixes

The "fixed-clock" value referenced in the documentation is actually a
compatible value, not a property name. Fix that.

Harden the "expected to have a clock-frequency property" language to
use "must" instead of "expected". The scripts error out if a node with
fixed-clock compatible is missing a clock-frequency property; it's not
a soft expectation.

Be explicit about clock-frequency units.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>