dt-bindings: net: dsa: microchip: Update ksz device tree bindings for drive strength

Extend device tree bindings to support drive strength configuration for the
ksz* switches. Introduced properties:
- microchip,hi-drive-strength-microamp: Controls the drive strength for
  high-speed interfaces like GMII/RGMII and more.
- microchip,lo-drive-strength-microamp: Governs the drive strength for
  low-speed interfaces such as LEDs, PME_N, and others.
- microchip,io-drive-strength-microamp: Controls the drive strength for
  for undocumented Pads on KSZ88xx variants.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>