Some PHY require to wait for a bit after the reset GPIO has been
toggled. This adds support for the DT property `phy-reset-post-delay`
which gives the delay in milliseconds to wait after reset.

If the DT property is not given, no delay is observed. Post reset
delay greater than 1000ms are invalid.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>