Add new helper routines for sdhci bridge drivers that use gpio pins for

Add new helper routines for sdhci bridge drivers that use gpio pins for
card presence and write protect switch detection.

A bridge driver just needs to call the setup routine in its attach(), the
teardown in its detach(), and write a couple tiny glue functions to connect
the sdhci interface functions to the new helper functions.  This is not
extensively documented, but multiple examples will exist real soon.