3.8: arm: rename am335x-bonelt.dts -> am335x-boneblack.dts

Rename am335x-bonelt.dts to am335x-boneblack.dts to reflect the actual
board name. Change all references to the dts file to match the new name.
Requires matching change in the U-Boot am335x_evm.h config.

Signed-off-by: Matt Porter <mporter@ti.com>