Add support for the Startech PEX1XS1PMINI

Add support for the Startech PEX1XS1PMINI

It has a smaller footprint than the already supported MPEX2S952

Change-Id: Ie36b67f9628882d516ca34ff164f0e8918955a5b
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Tested-by: Stefan Reinauer <reinauer@google.com>
Reviewed-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: http://review.coreboot.org/690
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>