dt-bindings: firmware: Add support for tegra186-bpmp DRAM MRQ GSCs

Add memory-region property to the tegra186-bpmp binding to support
DRAM MRQ GSCs.

Co-developed-by: Stefan Kristiansson <stefank@nvidia.com>
Signed-off-by: Stefan Kristiansson <stefank@nvidia.com>
Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>