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>

devices/Cargo.toml

@@ -11,6 +11,7 @@ arch = { path = "../arch" }
 bitflags = "1.3.2"
 byteorder = "1.4.3"
 epoll = "4.3.1"
+hypervisor = { path = "../hypervisor" }
 libc = "0.2.126"
 log = "0.4.17"
 versionize = "0.1.6"