mmc: SPI-mode support for SD cards.

Introduce SPI-mode support which allows an SD card to communicate with a
host system using SPI protocol, as described in the SD Card Specification.

This feature is useful for low-end, FPGA or RISC-V research systems when a
SoC is limited in terms of peripherals available (e.g. lack of a dedicated
MMC controller in hardware). Examples of such systems include Codasip,
lowRISC and CVA6.

Project timeline:
2007: Warner first discussed SPI operational mode in his MMC presentation:

https://people.freebsd.org/~imp/bsdcan2007.pdf

2012: Patrick Kelsey engineered the support.
2025: Ruslan cleaned up, tested on Codasip X730 platform (RISC-V FPGA)

and put the patch to review.

2025: Patrick Kelsey reviewed the patch and aligned with the current MMC

code.

Reviewed by: pkelsey
Sponsored by: UKRI
Differential Revision: https://reviews.freebsd.org/D49248