generic: update pending PCS patch with .fill_available_pcs OP

generic: update pending PCS patch with .fill_available_pcs OP

While implementing standalone PCS support for DSA, it was found that making
the MAC driver passing the available_pcs array is limiting and problematic
for memory handling and allocation. To better handle this, change the logic
and make phylink allocate the struct and make the MAC driver implement a
function in phylink_config .fill_available_pcs to fill the PCS array.

Update the Airoha and Mediatek driver to reflect this new implementation.

Link: https://github.com/openwrt/openwrt/pull/23413
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>