mfd: rtsx: Add dma transfer function

rtsx driver using a single function for transfer data, dma map/unmap are
placed in one fix function. We need map/unmap dma in different place(for
mmc async driver), so add three function for dma map, dma transfer and
dma unmap.

Signed-off-by: Micky Ching <micky_ching@realsil.com.cn>
Signed-off-by: Lee Jones <lee.jones@linaro.org>