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>

arch/sh/boards/Makefile

@@ -5,4 +5,4 @@ obj-$(CONFIG_SH_AP325RXA)	+= board-ap325rxa.o
 obj-$(CONFIG_SH_MAGIC_PANEL_R2)	+= board-magicpanelr2.o
 obj-$(CONFIG_SH_RSK7203)	+= board-rsk7203.o
 obj-$(CONFIG_SH_SH7785LCR)	+= board-sh7785lcr.o
-obj-$(CONFIG_SH_SHMIN)		+= board-shmin..o
+obj-$(CONFIG_SH_SHMIN)		+= board-shmin.o