net: phy: air_phy_lib: Factorize BuckPBus register accessors

net: phy: air_phy_lib: Factorize BuckPBus register accessors

In preparation of Airoha AN8801R PHY support, move the BuckPBus
register accessors and definitions, present in air_en8811h driver,
into the Airoha PHY shared code (air_phy_lib), so they will be usable
by the new driver without duplicating them.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Louis-Alexis Eyraud <louisalexis.eyraud@collabora.com>
Link: https://patch.msgid.link/20260526-add-airoha-an8801-support-v5-3-01aea8dee69b@collabora.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>