net: phy: air_en8811h: Add clk provider for CKO pin

EN8811H outputs 25MHz or 50MHz clocks on CKO, selected by GPIO3.
CKO clock operates continuously from power-up through md32 loading.
Implement clk provider driver so we can disable the clock output in case
it isn't needed, which also helps to reduce EMF noise

Signed-off-by: Lucien.Jheng <lucienx123@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250409150902.3596-1-lucienx123@gmail.com


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