mmc: sdhci: remove platform_suspend/platform_resume callbacks

The only user (sdhci-of-esdhc) no longer uses these callbacks, so lets
remove them to discourage any further use.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>