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.