ARM: EXYNOS: Add #dma-cells for generic dma binding support for PL330

This patch adds #dma-cells property to PL330 DMA controller
nodes for supporting generic dma dt bindings on samsung exynos
platforms. #dma-channels and #dma-requests are not required now
but added in advance.

Signed-off-by: Padmavathi Venna <padma.v@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>