sh: export get_cpu_subtype

This patch fixes the following build error:

<--  snip  -->

...
  MODPOST 1837 modules
ERROR: "get_cpu_subtype" [arch/sh/oprofile/oprofile.ko] undefined!
...
make[2]: *** [__modpost] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>