sh: Disable 4kB pages on extended mode TLB.

4kB pages are unstable on extended mode TLB, it's recommended
that TLB compat mode be used when using a 4kB PAGE_SIZE. Set
the default for extended mode to 8kB.

This should have negligible impact, as other than the extra swap
cache entry bits, there's no reason to use the extended mode TLB
with 4kB pages.

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