Draft hardware changes for SPI - SD card integration

[elliotb-lowrisc]

Create the changes required to connect the Genesys2 microSD card slot and Verilator SD card DPI model to the Mocha SPI Host block (rather than loopback).

[elliotb-lowrisc]

Changes drafted in #446
Merge blocked by the need for a successful test (see other sub-issues of parent issue)