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.
[automated] Merge in upstream-github/stable/9