MAINTAINERS: Add myself as the reviewer for PCI Endpoint Subsystem

I've been reviewing the patches related to PCI Endpoint Subsystem for
some time. So I'd like to add myself as the reviewer to get immediate
attention to the patches.

Cc: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>