clk: si521xx: Clock driver for Skyworks Si521xx I2C PCIe clock generators

Add driver for the Skyworks Si521xx PCIe clock generators. Supported models
are Si52144/Si52146/Si52147, tested model is Si52144. It should be possible
to add Si5213x series as well.

Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230118191521.15544-2-marex@denx.de
[sboyd@kernel.org: Make clk_ops const]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>