phy/marvell: add 88ec048 support

Marvell 88ec048 is a derivative of its 88e1121r device.  From the programmer's
perspective, the one major difference is the addition of an additional control
bit in Page 2 Register 16 - used to control the padding of odd nibble
preambles.

This patch adds support for this new device, while inheriting as much code as
possible from the existing 88e1121r implementation.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>