target-sh4: MMU: optimize UTLB accesses

With the current code, the QEMU TLB is setup to match the read/write
mode of the MMU fault. This means when read access is done, the page
is setup in read-only mode. When the page is later accessed in write
mode, an MMU fault happened, and the page is switch in write-only
mode. This flip-flop causes a lot of calls to the MMU code and slow
down the emulation.

This patch changes the MMU emulation, so that the QEMU TLB is setup
to match the UTLB protection key. This impressively increase the
speed of the emulation.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
