Documentation: x86 shadow ROM disable

Add documentation on disabling the SPI flash which is mapped (shadowed)
into the x86 address space at 0x000e0000 - 0x000fffff.

TEST=None

Change-Id: I1d94d84c6cade97886a3274a7e7403f7b3275c5a
Signed-off-by: Lee Leahy <Leroy.P.Leahy@intel.com>
Reviewed-on: https://review.coreboot.org/14112
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
2 files changed