doc: add dma-buf IOCTL code to table

The code 'b' is used for dma-buf IOCTLs.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221128133853.355645-1-contact@emersion.fr