mmc: renesas_sdhi: consolidate DMAC CONFIG options

The description in the Makefile is odd.  Fix the CONFIG selection
in a cleaner way.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>