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 <[email protected]>
Reviewed-by: Cédric Le Goater <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Cédric Le Goater <[email protected]>
Message-id: 20200623072723[email protected]
Signed-off-by: Peter Maydell <[email protected]>