defconfig: am437x_gp_evm: enable eth driver model

Enable eth driver model for am437x_gp_evm as cpsw supports
driver model.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

configs/am437x_gp_evm_defconfig

@@ -47,3 +47,4 @@ CONFIG_USB_GADGET_DOWNLOAD=y
 CONFIG_G_DNL_MANUFACTURER="Texas Instruments"
 CONFIG_G_DNL_VENDOR_NUM=0x0403
 CONFIG_G_DNL_PRODUCT_NUM=0xbd00
+CONFIG_DM_ETH=y