Squashed commit of the following:

commit 7fd1f0d78546fa0315f4077b779efdd884e5bd53
Author: William Douglas <william@rocklandscientific.com>
Date:   Tue May 15 13:19:31 2018 -0700

    Add support for the second SDMMC device.

    The second SDMMC device was already supported but
    the clock was never enabled.  This fixes that.