dt-bindings: watchdog: Document Exynos850 watchdog bindings

dt-bindings: watchdog: Document Exynos850 watchdog bindings

Exynos850 SoC has two CPU clusters:
  - cluster 0: contains CPUs #0, #1, #2, #3
  - cluster 1: contains CPUs #4, #5, #6, #7

Each cluster has its own dedicated watchdog timer. Those WDT instances
are controlled using different bits in PMU registers, new
"samsung,index" property is added to tell the driver which bits to use
for defined watchdog node.

Also on Exynos850 the peripheral clock and the source clock are two
different clocks. Provide a way to specify two clocks in watchdog device
tree node.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20211107202943.8859-3-semen.protsenko@linaro.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>