FreeBSD loader expects to have mmsz variable set by bootloader.
U-Boot behaviour is that if buffer size is not big enough to keep
whole memory map, assign the smallest correct buffer size to sz
and return error.
In other words U-Boot assumes that nobody will need mmsz value when buffer
is not filled with memory map, which is not true, so calculated pages value
was too big to allocate.
Solution: Simply assign default value to mmsz.