mainboard/{iwill,amd/serenget_*}: Fix ptr discards const qualifier

Change-Id: I22e55eb2b7fe06c416e5e4fd322045bc7031ed63
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7693
Tested-by: build bot (Jenkins)
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>