net: phy: Add phy_modify_mmd() and phy_modify_mmd_changed() from Linux

Add phy_modify_mmd()/phy_modify_mmd_changed() from Linux 5.1.y as of commit
b8554d4f7288f ("net: phy: add register modifying helpers returning 1 on change")
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>