power: regulator: scmi: define LOG_CATEGORY

Define LOG_CATEGORY to allow filtering with log command.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>