Some platforms have channels which are not available for normal use.
This information is currently passed though platform data in internal
BSP kernels. Once those platforms land, they'll need to configure them
appropriately, so we may as well add the infrastructure.

Cc: Dan Williams <djbw@fb.com>
Cc: Per Forlin <per.forlin@stericsson.com>
Cc: Rabin Vincent <rabin@rab.in>
Acked-by: NVinod Koul <vinod.koul@intel.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>