sh: default to extended TLB support.

All SH-X2 and SH-X3 parts support an extended TLB mode, which has been
left as experimental since support was originally merged. Now that it's
had some time to stabilize and get some exposure to various platforms,
we can drop it as an option and default enable it across the board.

This is also good future proofing for newer parts that will drop support
for the legacy TLB mode completely.

This will also force 3-level page tables for all newer parts, which is
necessary both for the varying page sizes and larger memories.

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