regulator: doc: add missing documentation for init_cb

regulator: doc: add missing documentation for init_cb

Add comment documenting introduced init_cb.
This solves the following warning when building the kernel documentation:

./include/linux/regulator/driver.h:435: warning: Function parameter or struct member 'init_cb' not described in 'regulator_desc'

Fixes: cfcdf395c21e ("regulator: core: add callback to perform runtime init")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20241023155120.6c4fea20@canb.auug.org.au/
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patch.msgid.link/20241023-regulator-doc-fixup-v1-1-ec018742ad73@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>