net: phy: provide phylib stubs for hardware timestamping operations

net/core/dev_ioctl.c (built-in code) will want to call phy_mii_ioctl()
for hardware timestamping purposes. This is not directly possible,
because phy_mii_ioctl() is a symbol provided under CONFIG_PHYLIB.

Do something similar to what was done in DSA in commit 5a17818682cf
("net: dsa: replace NETDEV_PRE_CHANGE_HWTSTAMP notifier with a stub"),
and arrange some indirect calls to phy_mii_ioctl() through a stub
structure containing function pointers, that's provided by phylib as
built-in even when CONFIG_PHYLIB=m, and which phy_init() populates at
runtime (module insertion).

Note: maybe the ownership of the ethtool_phy_ops singleton is backwards,
and the methods exposed by that should be later merged into phylib_stubs.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20230801142824.1772134-12-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>