dmaengine: sh: rcar-dmac: Add dma_pause operation

This patch adds dma_pause operation. This patch is based on
Muhammad Hamza Farooq's patch.

After this patch applied, an issue that the sh-sci driver with
high baud rate might cause data lost disappeared because the DMAC
is possible to transmit between [1] and [2] below, and then
the residue of [1] is not true:

In rx_timer_fn() of the sh-sci.c:
	dmaengine_pause();
	...
	dmaengine_tx_status();		/* [1] */
	...
	dmaengine_terminate_all();	/* [2] */

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>