Commit 0d6392d introduced a stripped
down device tree used for both white and black Beaglebone variants
that included the 256 MiB memory node from am335x-bone-common.dtsi.
This leads to the following error in the MLO:

    mmu: Critical Error: Can't request SDRAM region for ttb at 9fff4000

This patch removes the (for the Beaglebone black) invalid memory size
from the common device tree and instead registers the memory bank
manually in the board file.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>