Documentation: of: Document the bindings used by Juno R1 PCIe host bridge

ARM's Juno R1 board used PLDA XpressRICH3-AXI IP to implement a PCIe host
bridge. Introduce "plda" as vendor prefix for PLDA and document the DT
bindings for PLDA XpressRICH3-AXI IP as well as ARM's Juno R1.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>