Introduce helper functions stm32mp_register_secure_gpio() and
stm32mp_register_non_secure_gpio() for drivers to register a
GPIO pin as secure or non-secure.

These functions are stubbed when shared resource driver is not
embedded in the BL image so that drivers do not bother whether they
shall register or not their resources.

Change-Id: I1fe98576c072ae31f75427c9ac5c9f6c4f1b6ed1
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>