clk: mediatek: Fix rate and dependency of MT8173 clocks

Remove the dependency from clk_null, and give all root clocks a
typical rate, include clkph_mck_o, usb_syspll_125m and hdmitx_dig_cts.

dpi_ck was removed due to no clock reference to it.

Replace parent clock of infra_cpum with cpum_ck, which is an external
clock and can be defined in the device tree.

Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>