net: designware: support phy reset device-tree bindings

Add support for the snps,reset-gpio, snps,reset-active-low (optional) and
snps,reset-delays-us device-tree bindings. The combination of these
three define how the PHY should be reset to ensure it's in a sane state.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Simon Glass <sjg@chromium.org>