am335x_evm: add support for BeagleBone Black DT name

Cc: Matt Porter <mporter@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Matt Porter <mporter@ti.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Nishanth Menon <nm@ti.com>