sh/boards/Makefile typo fix

The following build error was caused by an obvious typo:

<--  snip  -->

...
  LD      arch/sh/mm/built-in.o
make[2]: *** No rule to make target `arch/sh/boards/board-shmin..o', needed by `arch/sh/boards/built-in.o'.  Stop.

<--  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>