Allow to override default kernel virtual address assignment on ARM

Each plaform performs virtual memory split between kernel and user space
and assigns kernel certain amount of memory space. However, is is sometimes
reasonable to change the default values. Such situation may happen on
systems where the demand for kernel buffers is high, many devices occupying
memory etc. This of course comes with the cost of decreasing user space
memory range so shall be used with care. Most embedded systems will not
suffer from this limtation but rather take advantage of this potential
since default behavior is left unchanged.

Submitted by: Wojciech Macek <wma@semihalf.com>
Reviewed by: imp
Obtained from: Semihalf