ARM machdep refactoring: rewritten MMU initialization code (which cre…

…ates

initial pagetables before bootstrapping pmap, set stacks, msgbuf, etc)
to be more versatile and easier to understand. Moved such routine into
arm/machdep.c file. Moved some frequently-used routines from individual
ports machdep files into arm/machdep.c file. Changed pmap_devmap entries
allocation: now virtual address of device mapping is allocated dynamically.
This change will free port developers from managing virtual address space
and make mappings more compact (only needed devices will be mapped
without gaps between them). Also, pmap_devmap entries has a name: if
pmap_devmap entry name will match device alias in .dts file, physical
address and size of mapping will be automatically aquired from FDT.