platform API: make MMIO dataspace cap accessible

In case of the GPU multiplexer, we need to delegate MMIO memory
to the framebuffer client in form of a managed dataspace. To be
able to attach a given Platform::Device::Mmio object to a region map
we need to access its capability.

Ref genodelabs/genode#4578

repos/os/include/platform_session/device.h

@@ -117,6 +117,8 @@ class Platform::Device::Mmio : Range, Attached_dataspace, public Genode::Mmio
 
 		template <typename T>
 		T *local_addr() { return reinterpret_cast<T *>(_local_addr()); }
+
+		Dataspace_capability cap() { return Attached_dataspace::cap(); }
 };