Documentation: x86 device tree processing and memory map

Add documentation on:
*  FSP Silicon Init
*  How to start the x86 device tree processing for ramstage
*  Disabling the PCI devices
*  Generic PCI device drivers
*  Memory map support

TEST=None

Change-Id: If8f729a0ea1d48db4d5ec1d4ae3ad693e9fe44f0
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/13718
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
3 files changed