DMA: PL08x: constify plchan->cd and plat->slave_channels

We no longer write to the channel data structure, so we can make it
const throughout the driver.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>