net: pxa168_eth: Provide phy_interface mode on platform_data

The PXA168 Ethernet IP support MII and RMII connection to its PHY.
Currently, pxa168 platform_data does not provide a way to pass that
and there is one user of pxa168 platform_data (mach-mmp/gplug).
Given the pinctrl settings of gplug it uses RMII, so add and pass
a corresponding phy_interface_t.

Tested-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>