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>

arch/arm64/configs/defconfig

@@ -360,7 +360,7 @@ CONFIG_MESON_GXL_PHY=m
 CONFIG_MICREL_PHY=y
 CONFIG_MICROSEMI_PHY=y
 CONFIG_AT803X_PHY=y
-CONFIG_REALTEK_PHY=m
+CONFIG_REALTEK_PHY=y
 CONFIG_ROCKCHIP_PHY=y
 CONFIG_VITESSE_PHY=y
 CONFIG_USB_PEGASUS=m