regmap: reg_default_cb for flat cache defaults

Merge series from "Sheetal ." <sheetal@nvidia.com>:

This series adds a reg_default_cb callback for REGCACHE_FLAT to provide
defaults for registers not listed in reg_defaults. Defaults are loaded
eagerly during regcache init and the callback can use writeable_reg to
filter valid addresses and avoid holes.

Tegra ASoC drivers set reg_default_cb and add writeable_reg filtering for
AHUB RX holes to prevent invalid addresses from being marked valid.