In order to avoid passing random/junky values to DMA/HW as well as to
allow simplifying memory initialization in individual drivers, change
dma_alloc_coherent() to guarantee that memory it returns is properly
zeroed out.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>