phylib: Use common page register definition for Marvell PHYs.

The definition of the Marvell PHY page register is not specific to
88E1121, so rename the macro to MII_MARVELL_PHY_PAGE, and use it
throughout.

Suggested-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Cc: Cyril Chemparathy <cyril@ti.com>
Cc: Arnaud Patard <arnaud.patard@rtp-net.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>