fix(st-pmic): add static const to pmic_ops

The static was found by sparse tool:
drivers/st/pmic/stm32mp_pmic.c:456:18: warning: symbol 'pmic_ops'
 was not declared. Should it be static?
The const was also missing.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ibb5cfaf67ac980bf0af27712a95dbef05b617c25