misc: Add a pca9554 GPIO device model

Specs are available here:

    https://www.nxp.com/docs/en/data-sheet/PCA9554_9554A.pdf

This is a simple model supporting the basic registers for GPIO
mode.  The device also supports an interrupt output line but the
model does not yet support this.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Glenn Miles <milesg@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

include/hw/misc/pca9554.h

@@ -0,0 +1,36 @@
+/*
+ * PCA9554 I/O port
+ *
+ * Copyright (c) 2023, IBM Corporation.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+#ifndef PCA9554_H
+#define PCA9554_H
+
+#include "hw/i2c/i2c.h"
+#include "qom/object.h"
+
+#define TYPE_PCA9554 "pca9554"
+typedef struct PCA9554State PCA9554State;
+DECLARE_INSTANCE_CHECKER(PCA9554State, PCA9554,
+                         TYPE_PCA9554)
+
+#define PCA9554_NR_REGS 4
+#define PCA9554_PIN_COUNT 8
+
+struct PCA9554State {
+    /*< private >*/
+    I2CSlave i2c;
+    /*< public >*/
+
+    uint8_t len;
+    uint8_t pointer;
+
+    uint8_t regs[PCA9554_NR_REGS];
+    qemu_irq gpio_out[PCA9554_PIN_COUNT];
+    uint8_t ext_state[PCA9554_PIN_COUNT];
+    char *description; /* For debugging purpose only */
+};
+
+#endif