dt-bindings: net: ethernet-phy: Add timing-role role property for ethernet PHYs

This patch introduces a new `timing-role` property in the device tree
bindings for configuring the master/slave role of PHYs. This is
essential for scenarios where hardware strap pins are unavailable or
incorrectly configured.

The `timing-role` property supports the following values:
- `forced-master`: Forces the PHY to operate as a master (clock source).
- `forced-slave`: Forces the PHY to operate as a slave (clock receiver).
- `preferred-master`: Prefers the PHY to be master but allows negotiation.
- `preferred-slave`: Prefers the PHY to be slave but allows negotiation.

The terms "master" and "slave" are retained in this context to align
with the IEEE 802.3 standards, where they are used to describe the roles
of PHY devices in managing clock signals for data transmission. In
particular, the terms are used in specifications for 1000Base-T and
MultiGBASE-T PHYs, among others. Although there is an effort to adopt
more inclusive terminology, replacing these terms could create
discrepancies between the Linux kernel and the established standards,
documentation, and existing hardware interfaces.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Divya Koppera <divya.koppera@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>