vendorcode/amd/agesa/f1{5,5tn,6kb}: Silence empty loop warn

Add decorations to specify that empty loop is intended so.

Change-Id: Ia3e40d341eca5e26da3832edc733cf1ccc96c136
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Found-by: Clang
Reviewed-on: http://review.coreboot.org/7688
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>