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.