net: phy: Add MDIO PCS 2.5G and 5G speed macros from Linux

Add MDIO PCS 2.5G and 5G speed macros from Linux 5.1.y as of commit
7fd8afa8933a0 ("net: phy: Add generic support for 2.5GBaseT and 5GBaseT")
This is used by the upcoming Marvell 10G PHY driver.

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