region->end is defined as an upper bound of the requested address range,
exclusive --- i.e. as an address outside of the range in which the
requested CSR is to be placed.

Hence 0x0001,0000,0000,0000 is the biggest valid region->end, not
0x0000,ffff,ffff,fffc like the current check asserted.

For simplicity, the fix drops the region->end & 3 test because there is
no actual problem with these bits set in region->end.  The allocated
address range will be quadlet aligned and of a size of multiple quadlets
due to the checks for region->start & 3 and handler->length & 3 alone.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>