hw/char/cmsdk-apb-uart.c: Implement CMSDK APB UART

Implement a model of the simple "APB UART" provided in
the Cortex-M System Design Kit (CMSDK).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1500029487-14822-3-git-send-email-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>