spi kerneldoc update

This adds kerneldoc to the SPI framework.  The "spi_driver" and
"spi_board_info" structs were previously not described.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>