Fix condition where bootm_size not set and wrong memory size reported

If the user sets bootm_low and does not set bootm_size, u-boot will
report the memory node in the flat device tree incorrectly. Instead
of reporting the remaining size of memory, it will report the total
available memory which is incorrect.

Specifically this fixes the situation when booting a relocatable
kernel and the memory is reported as an offset and size in the
device tree, and the size needs to be adjusted accordingly.

Signed-off-by: Matthew McClintock <msm@freescale.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>