ARM: AM335x: Beaglebone: Use stripped down devicetree for MLO
The beaglebone white and black have different devicetrees. Both
get linked into the MLO which makes it too big. Use a devicetree
generated from am335x-bone-common.dtsi which both board variants
can share. This reduces the binary size by about 30k.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>