kexec/ppc: cleanup crash regions creation

Move filling crash_memory_range table entries into a separate routine,
which saves quite a few lines of code.

In this routine, if range spawns over lowmem-highmem border, split range
into two. This is needed to get proper virtual address for lowmem part.
Similar thing is already done for x86. Credits to Yadviga Grigorieva
<yadviga@dev.rtsoft.ru> for tracking down this issue for ppc.

Also this patch makes excluding crash kernel regoin a bit shorter, and
removes unused variable to get rid of compiler warning.

Signed-off-by: Nikita Yushchenko <nyushchenko@dev.rtsoft.ru>
Signed-off-by: Simon Horman <horms@verge.net.au>