net: phy: fixed_phy: transition to the faux device interface

net: phy: fixed_phy: transition to the faux device interface

The net fixed phy driver does not require the creation of a platform
device. Originally, this approach was chosen for simplicity when the
driver was first implemented.

With the introduction of the lightweight faux device interface, we now
have a more appropriate alternative. Migrate the device to utilize the
faux bus, given that the platform device it previously created was not
a real one anyway. This will get rid of the fake platform device.

Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://patch.msgid.link/20250319135209.2734594-1-sudeep.holla@arm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>