MAINTAINERS: Add Jim Quinlan et al as Broadcom STB PCIe maintainers

Add Jim Quinlan, Nicolas Saenz Julienne, and Florian Fainelli as
maintainers of the Broadcom STB PCIe controller driver.

This driver is also included in these entries:

  BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE
  BROADCOM BCM7XXX ARM ARCHITECTURE

which cover the Raspberry Pi specifics of the PCIe driver.

Link: https://lore.kernel.org/r/20210818225031.8502-1-jim2101024@gmail.com
Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>