regulator: stpmic1_regulator: Properly document 'struct stpmic1_regulator_cfg'

In kerneldoc format, data structures have to start with 'struct'
else the kerneldoc tooling/parsers/validators get confused.

Fixes the following W=1 kernel build warning:

 drivers/regulator/stpmic1_regulator.c:25: warning: cannot understand function prototype: 'struct stpmic1_regulator_cfg '

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Pascal Paillet <p.paillet@st.com>
Link: https://lore.kernel.org/r/20200626065738.93412-5-lee.jones@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>