sh dreamcast: export board_pci_channels

This patch fixes the following build error:

<--  snip  -->

...
  MODPOST 1837 modules
ERROR: "board_pci_channels" [drivers/pcmcia/yenta_socket.ko] undefined!
...
make[2]: *** [__modpost] Error 1

<--  snip  -->

I freely admit that it's a pathological configuration, but as long as
it is allowed it should build.

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