pinctrl: samsung: Document hidden requirement about one external wakeup

pinctrl: samsung: Document hidden requirement about one external wakeup

Hardware (S5Pv210 and all Exynos SoCs) provides only 32 external
interrupts which can wakeup device from deep sleep modes.  On S5Pv210
these are gph0-gph3.  On all 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.

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>
Change-Id: I69ced8e994ab60b85ef509b59d50bf2f4589cf1d