Add a new sdhci interface method, get_card_present().

Many embedded SoC controllers that are (more or less) sdhci-compatible don't
implement card detect, and the related values in the PRESENT_STATE register
aren't useful. A bridge driver can now implement get_card_present() to read
a gpio pin or whatever else is necessary for that system.

The default implementation reads the CARD_PRESENT bit from the PRESENT_STATE
register, so existing drivers will keep working (or keep not-fully-working,
since many drivers right now can't detect card insert/remove).