misc/pca9552: Add qom set and get

misc/pca9552: Add qom set and get

Following the pattern of the work recently done with the ASPEED GPIO
model, this adds support for inspecting and modifying the PCA9552 LEDs
from the monitor.

 (qemu) qom-set  /machine/unattached/device[17] led0 on
 (qemu) qom-set  /machine/unattached/device[17] led0 off
 (qemu) qom-set  /machine/unattached/device[17] led0 pwm0
 (qemu) qom-set  /machine/unattached/device[17] led0 pwm1

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20200114103433.30534-6-clg@kaod.org
[clg: - removed the "qom-get" examples from the commit log
      - merged memory leak fixes from Joel ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>