soc/amd/common/block/gpio_banks: add comment about acpimmio_* symbols

Suggested-by: Martin Roth <martinroth@chromium.org>
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I0016a6c7d6581cb261cab6178268c1a86b89c839
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56831
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Jason Glenesk <jason.glenesk@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/common/block/gpio_banks/gpio.c

@@ -15,6 +15,12 @@
 #include <assert.h>
 #include <string.h>
 
+/*
+ * acpimmio_gpio0, acpimmio_remote_gpio and acpimmio_iomux are defined in
+ * soc/amd/common/block/acpimmio/mmio_util.c and declared as extern variables/constants in
+ * amdblocks/acpimmio.h which is included in this file.
+ */
+
 /* MMIO access of new-style GPIO bank configuration registers */
 static inline void *gpio_ctrl_ptr(gpio_t gpio_num)
 {