drm/amd/display: dc/clk_mgr: add support for SI parts (v2)

drm/amd/display: dc/clk_mgr: add support for SI parts (v2)

(v1) Changelog

[Why]
After commit c69dd2d "drm/amd/display: Refactor clk_mgr functions"
dc/clk_mgr requires these changes to add SI parts support
Necessary to avoid hitting default: ASSERT(0); /* Unknown Asic */
that would cause kernel freeze

[How]
Add case statement for FAMILY_SI chipsets

(v2) Changelog

[Why]
DCE6 has no DPREFCLK_CNTL register

[How]
Add DCE6 specific macros definitions for CLK registers and masks
Add DCE6 specific dce60/dce60_clk_mgr.c for DCE6 customization
Code style: reuse all the public functions in dce100/dce_clk_mgr.h header
Code style: use dce60_* static functions as per other DCE implementations
Add dce60_get_dp_ref_freq_khz() w/o using DPREFCLK_CNTL register
Use dce60_get_dp_ref_freq_khz() function in dce60_funcs
Add DCE6 specific dce60_clk_mgr_construct
dc/clk_mgr/dce_clk_mgr.c: use dce60_clk_mgr_construct for FAMILY_SI chipsets
Add Makefile rules for dce60_clk_mgr.o target conditional to CONFIG_DRM_AMD_DC_SI

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>