hw/char/pl011: Add support for loopback

hw/char/pl011: Add support for loopback

This patch adds loopback for sent characters, sent BREAK,
and modem-control signals.

Loopback of send and modem-control is often used for uart
self tests in real hardware but missing from current pl011
model, resulting in self-test failures when running in QEMU.

This implementation matches what is observed in real pl011
hardware placed in loopback mode:
1. Input characters and BREAK events from serial backend
   are ignored, but
2. Both TX characters and BREAK events are still sent to
   serial backend, in addition to be looped back to RX.

Signed-off-by: Tong Ho <tong.ho@amd.com>
Signed-off-by: Francisco Iglesias <francisco.iglesias@amd.com>
Message-id: 20240227054855.44204-1-tong.ho@amd.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>