drm/amd/display: dc/gpio: add support for DCE6 (v2)

[Why]
hw_factory.c requires changes for DCE6 support

[How]
DCE6 targets added replicating and adapting existing DCE8 implementation.

(v2) changes due to following commit:
    91db9311 ("drm/amd/display: refactor gpio to allocate hw_container in constructor")

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>