MIPS: merge __ioremap_mode into ioremap_prot

There is no reason to have two ioremap with flags interfaces.  Merge
the historic mips __ioremap_mode into ioremap_prot which is a generic
kernel interface.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>