if_awg: Add support for allwinner,{tx,rx}-delay-ps bindings

Split out delay parsing into a separate function; we'll support both
{tx,rx}-delay as well as the new versions.

While here, validate that they're within the expected range and fail to
attach if they are not. Assuming that we can clamp the delay is a bad idea
that might result in a non-working awg anyways, so we'll fail early to make
it easier to catch.

This version also unsets the tx and rx delay registers unconditionally and
then sets them if we read a non-zero delay. These delay properties should
default to 0 if not specified, as declared in the binding documentation.
Presumably the delays will be set via hardware configuration if they're not
explicitly set in FDT.