dt-bindings: clk: si521xx: Add Skyworks Si521xx I2C PCIe clock generators

Add binding for Skyworks Si521xx PCIe clock generators. This binding
is designed to support Si52144/Si52146/Si52147 series I2C PCIe clock
generators, tested model is Si52144. It should be possible to add
Si5213x series as well.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230118191521.15544-1-marex@denx.de
Signed-off-by: Stephen Boyd <sboyd@kernel.org>