sh: Limit ioremap_prot() to 32bit pgprot parts.

Presently ioremap_prot() uses an unsigned long to pass the pgprot value
around. This results in the upper half of the pgprot being chomped when
using 64-bit pgprots on a 32-bit ABI (X2TLB and SH-5).

As the only users of ioremap_prot() are presently legacy parts, this
doesn't cause too much of an issue. In the future when the interface is
converted to use pgprot_t directly this can be re-enabled for the other
parts, too.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>