[ARM] Clean up discontigmem support

Most architectures have fairly simple discontiguous memory - a
simple set of successive regions each containing some memory.
These can be described simply as a log2 of their maximum size,
along with the base address of the first region and the number
of regions.

The base address is already described by PHYS_PFN_OFFSET, and
the number of regions via the MAX_NUMNODES and the number of
online nodes.

If we then supply the log2 of their maximum size, all the other
discontigmem macros can move into generic code.

There is one exception: lh7a40x seems to have a more complicated
setup; this is left alone.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>