MAINTAINERS: Orphan Synopsys DesignWare xData traffic generator

Gustavo Pimentel <gustavo.pimentel@synopsys.com> is listed as the
maintainer of the Synopsys DesignWare xData traffic generator, but he's no
longer at Synopsys, and nobody has stepped up to maintain it.

Mark Synopsys DesignWare xData traffic generator as orphaned and add it to
Gustavo's entry in CREDITS.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>