hw/misc: add a toy i2c echo device

hw/misc: add a toy i2c echo device

Add an example I2C device to demonstrate how a slave may master the bus
and send data asynchronously to another slave.

The device will echo whatever it is sent to the device identified by the
first byte received.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
[ clg: integrated fixes :
  https://lore.kernel.org/qemu-devel/Y3yMKAhOkYGtnkOp@cormorant.local/ ]
Message-Id: <20220601210831.67259-7-its@irrelevant.dk>
Signed-off-by: Cédric Le Goater <clg@kaod.org>