sh: Prevent 64-bit pgprot clobbering across ioremap implementations.

Presently 'flags' gets passed around a lot between the various ioremap
helpers and implementations, which is only 32-bits. In the X2TLB case
we use 64-bit pgprots which presently results in the upper 32bits being
chopped off (which handily include our read/write/exec permissions).

As such, we convert everything internally to using pgprot_t directly and
simply convert over with pgprot_val() where needed. With this in place,
transparent fixmap utilization for early ioremap works as expected.

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