dmaengine: shdma: add chcr_write/read function

CHCR register position is not same in all DMAC.
This patch adds new "chcr_offset" to decide it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>