mmc: sdhci: allow sdio interrupts while sdhci runtime suspended

Allow SDIO interrupts to be received while the SDHCI host is runtime
suspended.  We do this by leaving the AHB clock enabled while the
host is runtime suspended so we can access the SDHCI registers, and
so read and raise the SDIO card interrupt.

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>