ARM: configs: Enable FIXED_PHY in multi_v7 defconfig

CONFIG_FIXED_PHY is needed to have Ethernet working on STi boards.

Select it as built-in since RootFS is accessible from NFS on these boards.

Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>