clocksource/drivers/timer-tegra186: Reserve and service a kernel watchdog

Tegra SoCs supports multiple watchdog timers. If the kernel crashes or
hangs before userspace enables a watchdog, the system cannot recover and
may remain bricked, e.g. after a failed OTA update. The driver currently
leaves all watchdogs disabled until userspace configures them.

Reserve first available watchdog as a kernel-only watchdog for Tegra186
and Tegra234. Arm it during probe (120s timeout) and keep it alive in
the driver IRQ handler. Do not register it to userspace. Other available
watchdogs remain exposed to userspace. This guarantees the system can
reset itself in case of a hang or crash even when userspace never starts.

Signed-off-by: Kartik Rajput <kkartik@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@kernel.org>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Link: https://patch.msgid.link/20260507154557.2082697-5-kkartik@nvidia.com

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