arm: dts: mt7623: enable flow control on port@6

Flow control needs to be enabled on both sides to work.
It is already enabled on gmac0, enable it on port@6 too.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/r/20230210182505.24597-5-arinc.unal@arinc9.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>