Update gpio.md

Adding a note to explain that the pinmux must be used before attempting to the use the GPIO on the Raspberry Pi's header.
lowRISC · Jan 2, 2025 · 4e96f29 · 4e96f29
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diff --git a/doc/ip/gpio.md b/doc/ip/gpio.md
@@ -72,6 +72,8 @@ The input and output registers have the same bit mapping.
 
 When the output enable is set to zero it instead acts as an input pin.
 
+Note: Before using the Raspberry Pi HAT header's GPIO you should use the pinmux to configure them as input or output.
+
 ## Arduino Shield
 
 The Arduino Shield header has 14 pins that can act as GPIO.