net: phy: bcm84881: add BCM84891/BCM84892 support

net: phy: bcm84881: add BCM84891/BCM84892 support

The BCM84891 and BCM84892 are 10GBASE-T PHYs in the same family as the
BCM84881, sharing the register map and most callbacks. They add USXGMII
as a host interface mode.

bcm8489x_config_init() is separate from bcm84881_config_init(): it
allows only USXGMII (the only host mode available on the tested
hardware) and clears MDIO_CTRL1_LPOWER, which is set at boot on the
tested platform. Does not recur on ifdown/ifup, cable events, or
link-partner advertisement changes, so config_init is sufficient.

For USXGMII, read_status() skips the 0x4011 host-mode register: it
returns the same value regardless of negotiated copper speed (USXGMII
symbol replication). Speed comes from phy_resolve_aneg_linkmode() via
standard C45 AN resolution.

Tested on TRENDnet TEG-S750 (RTL9303 + 1x BCM84891 + 4x BCM84892)
running OpenWrt, where the MDIO controller driver is currently
OpenWrt-specific. Link verified at 100M, 1G, 2.5G, 10G.

Signed-off-by: Daniel Wagner <wagner.daniel.t@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20260330225310.2801264-1-wagner.daniel.t@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>