phy: marvell: phy-mvebu-a3700-comphy: Rename HS-SGMMI to 2500Base-X

Comphy phy mode 0x3 is incorrectly named. It is not SGMII but rather
2500Base-X mode which runs at 3.125 Gbps speed.

Rename macro names and comments to 2500Base-X.

Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: 9695375a ("phy: add A3700 COMPHY support")
Signed-off-by: David S. Miller <davem@davemloft.net>