FDT, clk_fixed: add gated-fixed-clock support and deferred initialization

[mmel]

Summary

• Add CLK_TYPE_GATED_FIXED with GPIO-based gate control.
• Support optional vdd-supply regulator.
• Delay clknode registration until parent clock and other dependencies are present (via new_pass).

While I'm in, switch license header to the new SPDX format.

MFC after: 2 weeks