net: phy: marvell: add downshift support for M88E1240

Add downshift support for 88E1240, it uses the same downshift
configuration registers as 88E1011.

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Link: https://lore.kernel.org/r/20210428095356.621536-1-fido_max@inbox.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>