MAINTAINERS: Add Sergio Paracuellos as MT7621 PCIe maintainer

Add myself as maintainer of the PCIe Controller driver for MT7621 SoCs.

Link: https://lore.kernel.org/r/20210922050035.18162-4-sergio.paracuellos@gmail.com
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>