clk: Add KUnit tests for assigned-clock-sscs

Add KUnit test coverage for the assigned-clock-sscs DT property that
configures spread spectrum on clocks before they are used.

Extend the existing test infrastructure to support spread spectrum:
- Add struct clk_spread_spectrum field to clk_dummy_context and a
  clk_dummy_set_spread_spectrum callback
- Wire set_spread_spectrum into all dummy clock ops
- Extend clk_assigned_rates_register_clk and test parameter struct
  to propagate initial SSCS values

Add a new separate test suite clk_assigned_sscs with three categories:

  1. clk_assigned_sscs_assigns_one — verifies that a single
     assigned-clock-sscs entry correctly configures spread spectrum
     on one clock, testing both provider and consumer paths

  2. clk_assigned_sscs_assigns_multiple — verifies that multiple
     assigned-clock-sscs entries configure spread spectrum on two
     clocks, testing both provider and consumer paths

  3. clk_assigned_sscs_skips — verifies that malformed DT properties
     are correctly skipped without error: missing assigned-clocks,
     zero-valued SSCS, and null phandles, tested for both provider
     and consumer scenarios

New DT overlays are added for all test scenarios:
  - kunit_clk_assigned_sscs_one{,consumer} — single valid entry
  - kunit_clk_assigned_sscs_multiple{,consumer} — two valid entries
  - kunit_clk_assigned_sscs_without{,consumer} — missing assigned-clocks
  - kunit_clk_assigned_sscs_zero{,consumer} — all-zero SSCS values
  - kunit_clk_assigned_sscs_null{,consumer} — null phandle

Co-developed-by: Brian Masney <bmasney@redhat.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Brian Masney <bmasney@redhat.com>

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