mainboard/siemens/sitemp_g1p1/mainboard.c: Remove unicode in string

Remove illegal character encoding in string literal.

Change-Id: I3c8dc67363705a2160e8266d1cea78c0d34d076f
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7713
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>