mr_canhubk3: document WKPU support

Add WKPU related documentation.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>

boards/arm/mr_canhubk3/doc/index.rst

@@ -48,6 +48,7 @@ Interface     Controller  Driver/Component
 SIUL2         on-chip     | pinctrl
                           | gpio
                           | external interrupt controller
+WKPU          on-chip     wake-up interrupt controller
 LPUART        on-chip     serial
 QSPI          on-chip     flash
 FLEXCAN       on-chip     can
@@ -69,6 +70,14 @@ Each GPIO port is divided into two banks: low bank, from pin 0 to 15, and high
 bank, from pin 16 to 31. For example, ``PTA2`` is the pin 2 of ``gpioa_l`` (low
 bank), and ``PTA20`` is the pin 4 of ``gpioa_h`` (high bank).
 
+Some GPIO pins support to generate external interrupts and/or wake-up interrupts.
+To configure a GPIO to trigger wake-up interrupts, follow the instructions in
+:zephyr_file:`dts/bindings/gpio/nxp,s32-gpio.yaml`.
+
+.. note::
+
+   Power-management functionalities are currently not supported on S32K3 SoCs.
+
 LEDs
 ----