early alloc change broke sh

Since commit 0fa1c579349fdd90173381712ad78aa99c09d38b (of/fdt: use
memblock_virt_alloc for early alloc), attempting to boot on sh (j2,
nommu) fails with OOM:

[    0.000000] bootmem alloc of 7836 bytes failed!
[    0.000000] Kernel panic - not syncing: Out of memory

I suspect there are significant differences in memblock_virt_alloc and
memblock_alloc (perhaps specific to nommu?). It looks like microblaze
was also affected:

http://lkml.iu.edu/hypermail/linux/kernel/1801.1/02200.html

I'll continue looking for a solution but I wanted to let you know
right away in case you might know what's wrong and have a fix. It
would be nice if we could get a fix for this regression in 4.17 since
multiple archs are broken.

Rich