Maximize available kva space by doing static device mapping from the top

Maximize available kva space by doing static device mapping from the top
of the address space downwards, and then returning the lowest mapped
device address from initarm_lastaddr().  This adds over 500MB of kva
space compared to the old way of hardcoding the end address as 0xE0000000.

Also, pre-map most of the SoC's common memory-mapped devices using 1MB
section mappings so that all device access uses just a few TLB entries.
Graphics devices aren't mapped this way yet, but probably should be.

To provide this new functionality without pasting identical code into
multiple imxNN_machdep.c files, rework the imx machdep code so that
things common to the whole family of SoCs are in a new imx_machdep.c file.
The rewritten imxNN_machdep.c files contain just things specific to an
individual SoC.