gpio: uapi: clarify hte references

The full name of the HTE subsystem is "hardware timestamping engine",
so correct references and highlight that this refers to the HTE
subsystem.

Extend the description of struct gpio_v2_line_event to clarify that
the timestamp_ns is sourced from the HTE if the
GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE is set.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>