hw: timer: Add ASPEED RTC device

hw: timer: Add ASPEED RTC device

The RTC is modeled to provide time and date functionality. It is
initialised at zero to match the hardware.

There is no modelling of the alarm functionality, which includes the IRQ
line. As there is no guest code to exercise this function that is
acceptable for now.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20190618165311.27066-4-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>