cw1200: Rework SDIO platform support to prevent build problems.

Based on discussions with And Bergmann, this patch changes the SDIO
platform code to default to supporting the Sagrad devices, allowing for
it to be overridden in board setup code.  This renders the cw1200_sagrad
module suplerflous, so it is now removed.

It also moves the documentation that was in the cw1200_sagrad source to
the platform header.

Signed-off-by: Solomon Peachy <pizza@shaftnet.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>