sb/amd/sb700: Disable LPC ROM mapping when SPI Flash is used

Do not map LPC ROM into the system memory space when SPI Flash
is configured instead of an LPC ROM.

This resolves a long-standing hard boot hang issue on the ASUS
KGPE-D16 and related systems; in a nutshell, the incorrectly
mapped LPC ROM overrode low memory required by ramstage, causing
decompressed ramstage layout-dependent vectoring to romstage code
and subsequent execution of random sections of romstage.  Sometimes
these random sections of romstage reconfigured the hardware in such
a way that it could not access SPI Flash on the next boot attempt.

Change-Id: I115e5d834f0ca99c2d9dbb5b9b5badbea1d98574
Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Reviewed-on: https://review.coreboot.org/19280
Tested-by: build bot (Jenkins)
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Daniel Kulesz <daniel.ina1@googlemail.com>

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