net: phy: at803x: add QCA9561 support

Add support for the embedded fast-ethernet PHY found on the QCA9561
WiSoC platform. It supports the usual Atheros PHY featureset including
the cable tester.

Tested on a Xiaomi MiRouter 4Q (QCA9561)

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David Bauer <mail@david-bauer.net>
Link: https://lore.kernel.org/r/20211005225401.10653-1-mail@david-bauer.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>