watchdog: Add driver for Gunyah Watchdog

watchdog: Add driver for Gunyah Watchdog

On Qualcomm SoCs running under the Gunyah hypervisor, access to watchdog
through MMIO is not available on all platforms. Depending on the
hypervisor configuration, the watchdog is either fully emulated or
exposed via ARM's SMC Calling Conventions (SMCCC) through the Vendor
Specific Hypervisor Service Calls space.

Add driver to support the SMC-based watchdog provided by the Gunyah
Hypervisor. Device registration is done in the QCOM SCM driver after
checks to restrict the watchdog initialization to Qualcomm devices
running under Gunyah.

Gunyah watchdog is not a hardware but an SMC-based vendor-specific
hypervisor interface provided by the Gunyah hypervisor. The design
involving QCOM SCM driver for registering the platform device has been
devised to avoid adding non-hardware nodes to devicetree.

Tested-by: Shivendra Pratap <shivendra.pratap@oss.qualcomm.com>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org>
Tested-by: Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Hrishabh Rajput <hrishabh.rajput@oss.qualcomm.com>
Signed-off-by: Pavankumar Kondeti <pavan.kondeti@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260311-gunyah_watchdog-v8-2-4c1c0689de22@oss.qualcomm.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>