regmap: add KUnit coverage for reg_default_cb callback

regmap: add KUnit coverage for reg_default_cb callback

Add a flat-cache KUnit test that verifies reg_defaults are honored while
missing entries are populated via the reg_default_cb callback without
hardware reads. This exercises the new callback path added for
REGCACHE_FLAT defaults.

Test: ./tools/testing/kunit/kunit.py run regmap
Result:
======== reg_default_callback_populates_flat_cache  ========
[PASSED] flat-default @0x0
[PASSED] flat-default fast I/O @0x0
[PASSED] flat-default @0x2001
==== [PASSED] reg_default_callback_populates_flat_cache ====

Signed-off-by: Sheetal <sheetal@nvidia.com>
Link: https://patch.msgid.link/20260123095346.1258556-5-sheetal@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>