net: pcs: xpcs: add xpcs_destroy_pcs() and xpcs_create_pcs_mdiodev()

net: pcs: xpcs: add xpcs_destroy_pcs() and xpcs_create_pcs_mdiodev()

Provide xpcs create/destroy functions that return and take a phylink_pcs
pointer instead of an xpcs pointer. This will be used by drivers that
have been converted to use phylink_pcs pointers internally, rather than
dw_xpcs pointers.

As xpcs_create_mdiodev() no longer makes use of its interface argument,
pass PHY_INTERFACE_MODE_NA into xpcs_create_mdiodev() until it is
removed later in the series.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1svfMQ-005ZIL-Bi@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>