mibuild: define memory card pins of the Milkymist One platorm

This patch adds the memory card pins to the M1 platform definition in
mibuild.

I've tentatively named them "mmc". As far as I can tell, "MMC" is not
trademarked ("MultiMediaCard" the new "eMMC" would be), and "MMC" is
commonly used in the industry in a descriptive way to refer to this
sort of interface.

The original Verilog-based M1 calls the interface "mc", but since
several names have changed between milkymist and -ng, I thought I'd
use a more familiar name.

Usage example (clock signal divided by powers of two on the MMC TPs):
https://github.com/wpwrak/ming-ddc-debug/blob/counter-on-mmc/build.py

- Werner