Bump the maximum VM space from 3 * memory size to a fixed
256MB. That's all we have room for since we map the hardware registers
starting at 0xd0000000. This allows my 64MB AT91SAM9G20 to boot again
after the unmmaped I/O changes were MFC'd at r251897. Other
subplatforms may need similar treatment.
4096 is 4MB, and macb doesn't even come close to working, so don't say
it is an altenative. It hasn't worked in quite some time. The die
hards can still try to use macb in the tree if they want...
MFC:

r246881 | ian | 2013-02-16 13:43:16 -0700 (Sat, 16 Feb 2013) | 4 lines

In _bus_dmamap_addseg(), the return value must be zero for error, or the size
actually added to the segment (possibly smaller than the requested size if
boundary crossings had to be avoided).

This fixes NFS root on Atmel platforms, and likely others.