mmc: renesas_sdhi: break SCC reset into own function

renesas_sdhi_reset used to mainly reset the SCC but is now doing more
and even more will be added. So, factor out SCC reset to have a clear
distinction when we want to reset either SCC or SDHI+SCC.

Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210317091622.31890-3-wsa+renesas@sang-engineering.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>