clk: mediatek: add MT7981 clock support

Add MT7981 clock support, include topckgen, apmixedsys, infracfg and
ethernet subsystem clocks.

The drivers are based on clk-mt7981.c which can be found in MediaTek's
SDK sources. To be fit for upstream inclusion the driver has been split
into clock domains and the infracfg part has been significantly
de-bloated by removing all the 1:1 factors (aliases).

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/8136eb5b2049177bc2f6d3e0f2aefecc342d626f.1674703830.git.daniel@makrotopia.org


Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
[sboyd@kernel.org: Add module license]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>