MAINTAINERS: Add Gustavo Pimentel as DesignWare PCI maintainer

Currently I am managing the Synopsys drivers & tools team (full-time) and
so I am passing the pcie-designware maintenance to Gustavo.

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
CC: Jingoo Han <jingoohan1@gmail.com>