mmc: renesas-sdhi: make renesas_sdhi_sys_dmac main module file

Make renesas_sdhi_sys_dmac.c a top-level module file that makes use of
library code supplied by renesas_sdhi_core.c

This is in order to facilitate adding other variants of SDHI;
in particular SDHI using different DMA controllers.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
[Arnd: Fixed module build error]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>