mb/asus/p2b: Enable hardware monitor access via I/O on ISA bus

Set up a 8-byte I/O range at 0x290-0x297 as PIIX4's generic device 9,
which activates a chip select when this range is accessed.

On the P2B family it connects to the W83781D hardware monitor,
allowing access to it over the ISA bus, just like vendor firmware.

Apparently this does not work on p3b-f, but no ill effects observed
either.

TEST=On p2b-ls lm-sensors can detect chip and get readings over ISA.

Change-Id: Iaed1df7230359e94c580c305f4769c8bb4f5fce0
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41561
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/asus/p2b/mainboard.c

@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <device/pci_ops.h>
+#include <southbridge/intel/i82371eb/i82371eb.h>
+
+/**
+ * Mainboard specific enables.
+ *
+ * @param chip_info Ignored
+ */
+static void mainboard_init(void *chip_info)
+{
+	const pci_devfn_t px43 = PCI_DEV(0, 4, 3);
+	u32 reg;
+	/*
+	 * Set up an 8-byte generic I/O decode block at device 9.
+	 * This will be for W83781D hardware monitor.
+	 * Port 0x290 mask 0x007
+	 *
+	 * This should enable access to W83781D over the ISA bus.
+	 */
+	reg = pci_s_read_config32(px43, DEVRESB);
+	reg |= (0x290 | (0xe7 << 16));
+	pci_s_write_config32(px43, DEVRESB, reg);
+}
+
+struct chip_operations mainboard_ops = {
+	.init = mainboard_init
+};