configs: am43xx_evm: Enable USB support

spl_dm_init is failing as usb to be probed with the following error:
"omap_dwc3@483c0000: ret=-2"
Enable usb to make spl boot on am43xx devices

Reported-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>