net: dsa: lantiq_gswip: add vendor property to setup MII refclk output

net: dsa: lantiq_gswip: add vendor property to setup MII refclk output

Read boolean Device Tree property "maxlinear,rmii-refclk-out" and switch
the RMII reference clock to be a clock output rather than an input if it
is set.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://patch.msgid.link/947d14970f74f760e4a60c777aabee64e7e4f356.1762170107.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>