pinctrl: samsung: Document hidden requirement about one external wakeup

ARMv7 hardware (S5Pv210 and Exynos SoCs) provides only 32 external
interrupts which can wakeup device from deep sleep modes.  On S5Pv210
these are gph0-gph3.  On all ARMv7 Exynos designs these are gpx0-gpx3.
There is only one 32-bit register for controlling the external wakeup
interrupt mask (masking and unmasking waking capability of these
interrupts).

This lead to implementation in pinctrl driver and machine code which was
using static memory for storing the mask value and not caring about
multiple devices of pin controller... because only one pin controller
device will be handling this.

Since each pin controller node in Device Tree maps onto one device, this
corresponds to hidden assumption in parsing the Device Tree: external
wakeup interrupts can be defined only once.  Make this assumption an
explicit requirement.

ARMv8 Exynos5433 and Exynos7 bring additional 32 external wakeup
interrupts (up to 64 total, banks gpa0-gpa3 and gpf1-gpf5) and another
32-bit wide register for controlling them.  Existing code does not
support it but anyway these additional interrupts will be belonging to
the same pin controller device/node.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Sylwester Nawrocki <snawrocki@kernel.org>
Acked-by: Tomasz Figa <tomasz.figa@gmail.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>

Documentation/devicetree/bindings/pinctrl/samsung-pinctrl.txt

@@ -145,8 +145,13 @@ A. External GPIO Interrupts: For supporting external gpio interrupts, the
 
 B. External Wakeup Interrupts: For supporting external wakeup interrupts, a
    child node representing the external wakeup interrupt controller should be
-   included in the pin-controller device node. This child node should include
-   the following properties.
+   included in the pin-controller device node.
+
+   Only one pin-controller device node can include external wakeup interrupts
+   child node (in other words, only one External Wakeup Interrupts
+   pin-controller is supported).
+
+   This child node should include following properties:
 
    - compatible: identifies the type of the external wakeup interrupt controller
      The possible values are: