clk_fixed: Resolve parent before registering fixed-factor clocks

[jaewon8868_gmail.com]

Summary

This fixes a fixed-factor clock registration regression introduced by
e495998e7 (D58103).

Fixed-factor clocks are registered without resolving their parent,
passing parent_cnt = 1 and NULL parent_names to clknode_create(), which
can panic even when the parent is already available.

Resolve the parent before registration, and defer registration during
attach and subsequent bus passes while the parent is unavailable.

Fixes: e495998e7 ("FDT, clk_fixed: add gated-fixed-clock support and deferred initialization")
Related review: https://reviews.freebsd.org/D58103

Test Plan

Built arm64 GENERIC kernels from main d02883b3d before and after this
patch (NO_MODULES=yes). Boot-tested on QEMU virt using FDT (acpi=off)
and corrected DT overlays. All six runs matched the expected results:

• Before: clknode_create() panics with both available and unavailable parents.
• After: all four cases reach login. Ready and deferred parents produce a 24 MHz fixed-factor clock; a deferred chain produces 24/12/6 MHz; an unavailable provider leaves the child unregistered without panic.

Verified overlay application, clock registration, parent links and
frequencies using boot logs and sysctl hw.clock. Confirmed that the
booted kernel SHA256 matches the built and installed kernel.