arm64: configs: Select REALTEK_PHY as built-in

On the i.MX8MP EVK board there is a RTL8211 EThernet PHY and
a typical use case is to boot via NFS. In this case the Ethernet
PHY driver needs to be built-in, so select the driver as built-in
by default.

Reported-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>