mmc: core: enable support for the standard "wakeup-source" property

Though the mmc core driver should/will continue to support the legacy
"enable-sdio-wakeup" property to enable SDIO as the wakeup source, we
need to add support for the new standard property "wakeup-source".

This patch adds support for "wakeup-source" property in addition to the
existing "enable-sdio-wakeup" property.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>