sh: Fix hugetlbfs dependencies for SH-3 && MMU configurations.

The hugetlb dependencies presently depend on SUPERH && MMU while the
hugetlb page size definitions depend on CPU_SH4 or CPU_SH5. This
unfortunately allows SH-3 + MMU configurations to enable hugetlbfs
without a corresponding HPAGE_SHIFT definition, resulting in the build
blowing up.

As SH-3 doesn't support variable page sizes, we tighten up the
dependenies a bit to prevent hugetlbfs from being enabled. These days
we also have a shiny new SYS_SUPPORTS_HUGETLBFS, so switch to using
that rather than adding to the list of corner cases in fs/Kconfig.

Reported-by: Kristoffer Ericson <kristoffer.ericson@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>