net: mdio: move device reset functions to mdio_device.c

net: mdio: move device reset functions to mdio_device.c

The functions mdiobus_register_gpiod() and mdiobus_register_reset()
handle the mdio device reset initialization, which belong to
mdio_device.c.
Move them from mdio_bus.c to mdio_device.c, and rename them to match
the corresponding source file: mdio_device_register_gpio() and
mdio_device_register_reset().
Remove 'static' qualifiers and declare them in
drivers/net/phy/mdio-private.h (new header file).

Signed-off-by: Buday Csaba <buday.csaba@prolan.hu>
Link: https://patch.msgid.link/5f684838ee897130f21b21beb07695eea4af8988.1763473655.git.buday.csaba@prolan.hu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>