sh: Disable PMB for SH4AL-DSP CPUs.

While the PMB is available on SH-4A parts, SH4AL-DSP parts exclude it
altogether. As such, explicitly disable PMB support for these parts. If
this changes in the future for newer subtypes, this will have to be made
more fine-grained.

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