net: phy: Add support for PHY timing-role configuration via device tree

Introduce support for configuring the master/slave role of PHYs based on
the `timing-role` property in the device tree. While this functionality
is necessary for Single Pair Ethernet (SPE) PHYs (1000/100/10Base-T1)
where hardware strap pins may be unavailable or incorrectly set, it
works for any PHY type.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Divya Koppera <divya.koppera@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>