phylib: rename reset-(post-)delay-us to reset-(de)assert-us

As suggested by Rob Herring [1] rename the previously introduced
reset-{,post-}delay-us bindings to the clearer reset-{,de}assert-us

[1] https://patchwork.kernel.org/patch/10104905/

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>