Add Unimac mdio compatible string for the special BCM6846
variant.

This variant has a few extra registers compared to other
versions.
Suggested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/linux-devicetree/b542b2e8-115c-4234-a464-e73aa6bece5c@broadcom.com/Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patch.msgid.link/20241012-bcm6846-mdio-v1-2-c703ca83e962@linaro.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>