net: phy: mscc: Add support for PHY LED control

net: phy: mscc: Add support for PHY LED control

Add support for the PHY LED controller in the MSCC VSC85xx driver. The
implementation provides LED brightness and hardware control through the
LED subsystem and integrates with the standard 'netdev' trigger.

Introduce new register definitions for the LED behavior register
(MSCC_PHY_LED_BEHAVIOR = 30) and the LED combine disable bits, which
control whether LEDs indicate link-only or combined link and activity
status. Implement a helper, vsc8541_led_combine_disable_set(), to update
these bits safely using phy_modify().

Add support for LED brightness control and hardware mode configuration.
The new callbacks implement the standard LED class operations, allowing
user control through sysfs. The brightness control maps to PHY LED force
on/off modes. The hardware control get and set functions translate
between the PHY-specific LED mode encodings and the LED subsystem
TRIGGER_NETDEV_* rules.

The combine feature is managed automatically based on the selected
rules. When both RX and TX activity are disabled, the combine feature is
turned off, causing LEDs to indicate link-only status. When either RX or
TX activity is enabled, the combine feature remains active and LEDs
indicate combined link and activity.

Register the LED callbacks for all VSC85xx PHY variants so that the LED
subsystem can manage their indicators consistently. Existing device tree
LED configuration and default behavior are preserved.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20251112135715.1017117-4-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>