staging: tidspbridge: drop const from custom mmu implementation

Custom mmu functions receive a 'const void __iomem *', all the
callers pass a 'void __iomem *', so drop the const to fix the
warnings like:

warning: passing argument 2 of '__raw_writel' discards qualifiers from pointer target type
../io.h:88: note: expected 'volatile void *' but argument is of type 'const void *'

Signed-off-by: Omar Ramirez Luna <omar.ramirez@copitl.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>