clk: mediatek: add support for gate clock to reference topckgen clock

clk: mediatek: add support for gate clock to reference topckgen clock

Add support for gate clock get_rate to reference topckgen clock for
infracfg-ao implementation.

In infracfg-ao implementation topckgen is on second level of parent with
infracfg in the middle.
To correctly detect this, check the driver of the dev parent and use the
second level parent if it's not mtk_clk_topckgen.

Due to all the dependency, parent tree must be filled before a gate is
used, hence is safe to assume it will be there.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>