aarch64: Simplify GIC related structs definition

Combined the `GicDevice` struct in `arch` crate and the `Gic` struct in
`devices` crate.

After moving the KVM specific code for GIC in `arch`, a very thin wapper
layer `GicDevice` was left in `arch` crate. It is easy to combine it
with the `Gic` in `devices` crate.

Signed-off-by: Michael Zhao <michael.zhao@arm.com>

vmm/src/device_manager.rs

@@ -4326,10 +4326,6 @@ impl Pausable for DeviceManager {
         #[cfg(target_arch = "aarch64")]
         {
             self.get_interrupt_controller()
-                .unwrap()
-                .lock()
-                .unwrap()
-                .get_gic_device()
                 .unwrap()
                 .lock()
                 .unwrap()