mmc: sdhci-brcmstb: Add shutdown callback

Shutdown controller and disable it's clocks to insure max power
savings in S5 on systems that leave power on.

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Link: https://lore.kernel.org/r/20200113210706.11972-5-alcooperx@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>