dt-bindings: pci: Add DT docs for Brcmstb PCIe device

The DT bindings description of the Brcmstb PCIe device is described.  This
node can be used by almost all Broadcom settop box chips, using
ARM, ARM64, or MIPS CPU architectures.

Signed-off-by: Jim Quinlan <jim2101024@gmail.com>