hte: Add Tegra HTE test driver

The test driver uses IRQ and GPIO lines to timestamp using HTE
subsystem. The patch also adds compilation support in Kconfig and
Makefile.

Signed-off-by: Dipen Patel <dipenp@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

hte: Add Tegra194 HTE kernel provider

Tegra194 device has multiple HTE instances also known as GTE (Generic
Hardware Timestamping Engine) which can timestamp subset of SoC lines
and signals. This provider driver focuses on IRQ and GPIO lines and
exposes timestamping ability on those lines to the consumers through
HTE subsystem.

Also, with this patch, added:
- documentation about this provider and its capabilities at
Documentation/hte.
- Compilation support in Makefile and Kconfig

Signed-off-by: Dipen Patel <dipenp@nvidia.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers: Add hardware timestamp engine (HTE) subsystem

Some devices can timestamp system lines/signals/Buses in real-time
using the hardware counter or other hardware means which can give
finer granularity and help avoid jitter introduced by software
timestamping. To utilize such functionality, this patchset creates
HTE subsystem where devices can register themselves as providers so
that the consumers devices can request specific line from the
providers. The patch also adds compilation support in Makefile and
menu options in Kconfig.

The provider does following:
- Registers chip with the framework.
- Provides translation hook to convert logical line id.
- Provides enable/disable, request/release callbacks.
- Pushes timestamp data to HTE subsystem.

The consumer does following:
- Initializes line attribute.
- Gets HTE timestamp descriptor.
- Requests timestamp functionality.
- Puts HTE timestamp descriptor.

Signed-off-by: Dipen Patel <dipenp@nvidia.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>