Fix NanoPC-T6 USB-C serial console: built-in driver + clock/power workarounds

[Copilot]

Earlycon output works on the USB-C debug port (UART2 @ 0xfeb50000) but the kernel goes silent after ~1s. The ttyS2 console never registers because the 8250 DesignWare driver isn't available early enough, and clock/power gating kills the UART hardware before the driver probes.

Changes

• Force serial driver stack built-in — SERIAL_8250_DW, SERIAL_8250_CONSOLE, SERIAL_OF_PLATFORM, SERIAL_EARLYCON as =y via extraStructuredConfig. The rockchip flake's kernel inherits NixOS linux_latest defaults which may have these as modules.

• Prevent UART2 clock/power gating — clk_ignore_unused and pd_ignore_unused kernel params stop the clock framework from gating SCLK_UART2/PCLK_UART2 before the driver claims them.

• Bridge earlycon → ttyS2 handoff — keep_bootcon keeps the boot console active alongside the real console so no output is lost during transition.

• Enable serial getty — serial-getty@ttyS2 for interactive login on the debug port.

• Remove FIQ debugger backport — the vendor FIQ debugger driver doesn't compile against mainline 6.18; use standard ttyS2 instead of ttyFIQ0.

• Restore DT patch to upstream-compatible state — removed FIQ debugger DT node and uart2 disable; uart2 stays enabled per upstream NanoPC-T6 DTS with uart2m0_xfer pinctrl.

boot.kernelParams = [
  "earlycon=uart8250,mmio32,0xfeb50000"
  "keep_bootcon"
  "clk_ignore_unused"
  "pd_ignore_unused"
  "console=ttyS2,1500000"
  "console=tty1"
];

# in kernelPatches extraStructuredConfig:
SERIAL_8250 = yes;
SERIAL_8250_CONSOLE = yes;
SERIAL_8250_DW = yes;
SERIAL_OF_PLATFORM = yes;
SERIAL_EARLYCON = yes;

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