[ARM] ignore high memory with VIPT aliasing caches

VIPT aliasing caches have issues of their own which are not yet handled.
Usage of discard_old_kernel_data() in copypage-v6.c is not highmem ready,
kmap/fixmap stuff doesn't take account of cache colouring, etc.
If/when those issues are handled then this could be reverted.

Signed-off-by: Nicolas Pitre <nico@marvell.com>