hw/net/lan9118: Extract lan9118_phy

hw/net/lan9118: Extract lan9118_phy

A very similar implementation of the same device exists in imx_fec. Prepare for
a common implementation by extracting a device model into its own files.

Some migration state has been moved into the new device model which breaks
migration compatibility for the following machines:
* smdkc210
* realview-*
* vexpress-*
* kzm
* mps2-*

While breaking migration ABI, fix the size of the MII registers to be 16 bit,
as defined by IEEE 802.3u.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20241102125724.532843-2-shentey@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>