device: Add AArch64 RTC PL031 implementation

This commit adds the implementation for the AArch64 PL031
Real Time Clock (RTC) that provides a long time base counter. This
is achieved by generating an interrupt signal after counting a
programmed number of cycles of a real-time clock input. The AArch64
guest VM of the cloud-hypervisor will use this RTC to sync the time
in itself.

Signed-off-by: Henry Wang <Henry.Wang@arm.com>

devices/src/legacy/mod.rs

@@ -10,6 +10,8 @@ mod cmos;
 #[cfg(feature = "fwdebug")]
 mod fwdebug;
 mod i8042;
+#[cfg(target_arch = "aarch64")]
+mod rtc_pl031;
 mod serial;
 
 #[cfg(feature = "cmos")]
@@ -18,3 +20,6 @@ pub use self::cmos::Cmos;
 pub use self::fwdebug::FwDebugDevice;
 pub use self::i8042::I8042Device;
 pub use self::serial::Serial;
+
+#[cfg(target_arch = "aarch64")]
+pub use self::rtc_pl031::RTC;