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 <[e-mail]>
Closes: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Jerome Brunet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

include/linux/regulator/driver.h

···
  *               config but it cannot store it for later usage.
  *               Callback should return 0 on success or negative ERRNO
  *               indicating failure.
+ * @init_cb: Optional callback called after the parsing of init_data.
+ *           Allows the regulator to perform runtime init if necessary,
+ *           such as synching the regulator and the parsed constraints.
+ *           Callback should return 0 on success or negative ERRNO
+ *           indicating failure.
  * @id: Numerical identifier for the regulator.
  * @ops: Regulator operations table.
  * @irq: Interrupt number for the regulator.