sdcard: Initial support for SD cards on PCI SDHCI controllers on QEMU

This adds basic read/write support for SD cards emulated by QEMU.
This code is not expected to work on real hardware, because the
current controller and card initialization is not robust.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Add wiki documentation to repository

Add a docs/ directory that contains the contents of the SeaBIOS wiki
in markdown format.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
docs: Don't point to repo README files

Now that the README files have been simplified, don't point the wiki
pages to them.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
docs: Add info on MODE16/MODESEGMENT compile time flags

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
docs: Add page describing SeaBIOS final object linking

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>