sparc64: fix prototypes of reads[bwl]()

Conventions for readsl() are the same as for readl() - any __iomem
pointer is acceptable, both const and volatile ones being OK.  Same
for readsb() and readsw().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Andreas Larsson <andreas@gaisler.com> # Making sparc64 subject prefix