sh: dma: Remove unused functionality

dma_extend(), get_dma_info_by_name(), register_chan_caps(), and
request_dma_bycap() are unused.  Remove them, and all related code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/2beb81fdd7592a94329e3c9a6ba56959f6094019.1709326528.git.geert+renesas@glider.be


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>