Integrate u-boot-seco-rk/seco_2017.09_next_clk-si5351

Commit: https://git.seco.com/edgehog/bsp/rockchip/u-boot-seco-rk/-/commit/e9d78cf908f4993c0f0e046e066d4f33e22a6464

[DRIVER][CLK] Add Si5351a simplified driver (only CLK0 output)

This is a simplified implementation of the Si5351a driver. It's only capable of using CLK0 output.

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Commit: https://git.seco.com/edgehog/bsp/rockchip/u-boot-seco-rk/-/commit/26ba3942cefae4080b10e5e0bafed056e29c72b5

[DRIVER][CLK] Add Si5351a driver header file

The header file contains register numbers and division constants. We also define the crystal frequency here, in future would be better to read it from the devicetree.