xtensa: fix init_bootmem_node() argument order

The second argument to init_bootmem_node() is the PFN to place the
bootmem bitmap at and the third argument is the first PFN on the node.

This is currently backwards but never made any problems as both values
were always zero.

Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>