hw/gpio: pca9554: add PCA9536 support

The PCA9536 is a 4-bit I/O expander that's register-compatible with the
PCA9554 but only has 4 pins.  Rather than duplicating the whole driver,
make the existing PCA9554 model parameterizable and register PCA9536 as
a subtype.

Introduce a PCA9554Class with a pin_count property, and replace every
hard-coded PCA9554_PIN_COUNT reference in the driver with the class
value.  The reset function now computes the correct pin mask from
pin_count instead of assuming 0xFF.

Signed-off-by: Emmanuel Blot <emmanuel.blot@free.fr>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260709-catalina-upgrade-v1-15-82a63fead90c@free.fr
Signed-off-by: Cédric Le Goater <clg@redhat.com>