net: mvpp2: Drop PHY_INTERFACE_MODE_SGMII_2500 support

This mode does not seem to be well defined and used anywhere, remove support for it.
Based on discussion:
- 1000baseX does c37 AN of duplex+pause
- SGMII does AN of duplex+pause+speed, at lower speed bytes are repeated 10x/100x
- 2500baseX does not do AN, or does very different c73 AN
- SGMII 2500 behavior is unclear

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>