Increase maximum size of permanent high memory area.

Some XHCI controllers can request a significant chunk of reserved
memory for scratch pad buffers.  (At least one controller Intel
Haswell based controller has been seen to request 64KiB.)  Unused
memory is returned after POST completes, so it should be okay to
increase the maximum permanent high memory zone from 64K to 256K.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>