mmc: support for SPI bus type

• Introduce the bus_type accessor, which allows to skip card selection and timing settings in SPI mode;
• Add MMC_CMD_IS_APP flag for commands followed by the APP command, which is required by the upcoming MMCSPI driver.

Reviewed by: pkelsey
Sponsored by: UKRI
Differential Revision: https://reviews.freebsd.org/D49249