hw/misc/pca9552: Add generic PCA955xClass, parent of TYPE_PCA9552

hw/misc/pca9552: Add generic PCA955xClass, parent of TYPE_PCA9552

Extract the code common to the PCA955x family in PCA955xClass,
keeping the PCA9552 specific parts into pca9552_class_init().
Remove the 'TODO' comment added in commit 5141d4158cf.

Suggested-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20200623072723.6324-5-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>