net: phy: at803x: Use a helper to check for phy reset existence

The at803x family of devices are subjected to an errata that requires
hard-reseting the PHY upon link change.

That can only work if there's a physical reset line wired to the PHY,
which the driver checks by looking if there's a reset GPIO configured
for the MDIO device.

The reset may however be controlled through a reset controller, which
isn't accounted for in the errata handling.

Besides that, PHY drivers aren't expected to directly access the
mdiodev's resources directly, let's therefore wrap this with a phylib
helper, that uses a similar mdio helper to check for reset existence.

This was found in preparation for bus-level resource management for
better mdio scan support.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260715101355.88536-1-maxime.chevallier@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>