Merge from stable/10 r259380:

Merge from stable/10 r259380:

MFC r259038, r259039:

 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.

 Although not strictly required to boot a 64MB board, bump
 vm_max_virtual_address to be KERNVIRTADDR + 256MB. This allows some
 future shock protection since the KVA requirements have gone up since
 the unmapped changes have gone in, as well as preventing us from
 overlapping with the hardware devices, which we map at 0xd0000000,
 which we'd hit with anything more than 85MB...

Approved by:	re@ (gjb@)

git-svn-id: svn://svn.freebsd.org/base/releng/10.0@259746 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f