MIPS: Disable pte_special() for MIPS32 with RiXi

Commit 61cbfff4 ("MIPS: pte_special()/pte_mkspecial() support")
added a _PAGE_SPECIAL bit to the pgprot bits of our PTEs. Unfortunately
for MIPS32 configurations with RiXi support this pushed the number of
pgprot bits to 13. Since the PFN field in EntryLo begins at bit 12 this
results in us shifting the most significant bit of the physical address
beyond the end of the PTE, leading any mapped access to a physical
address above 2GB to incorrectly access an address 2GB lower than
intended.

For now, disable the pte_special() support for MIPS32 configurations
that support RiXi.

Fixes: 61cbfff4

 ("MIPS: pte_special()/pte_mkspecial() support")
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Dmitry Korotin <dkorotin@wavecomp.com>
Cc: linux-mips@vger.kernel.org