board/am57xx supports all boards based on am57xx. Rename the taget
as TARGET_AM57XX_EVM.
Fixes: 74cc8b097d ("board: ti: beagle_x15: Rename to indicate support for TI am57xx evms")
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>