MAINTAINERS: Add Manivannan Sadhasivam as DesignWare PCIe driver maintainer

Manivannan has been actively reviewing patches and testing changes
related to the DesignWare core driver and other DWC-based PCIe drivers
for a while now.

Add Manivannan as a maintainer for the Synopsys DesignWare driver to make
his role and contributions official.

Thank you Manivannan! For all the help with DWC!

Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>