mmc: sdhci-pltfm: Fix linking err for sdhci-brcmstb

The implementation of sdhci_pltfm_suspend() is only available when
CONFIG_PM_SLEEP is set, which triggers a linking error:

"undefined symbol: sdhci_pltfm_suspend" when building sdhci-brcmstb.c.

Fix this by implementing the missing stubs when CONFIG_PM_SLEEP is unset.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Fixes: 5b191dcb

 ("mmc: sdhci-brcmstb: Fix mmc timeout errors on S5 suspend")
Cc: stable@vger.kernel.org
Tested-By: Nicolas Schichan <nschichan@freeebox.fr>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>