feat(st-gpio): do not apply secure config in BL2

At boot, the devices under ETZPC control are secured, so should be
their GPIOs. As securable GPIOs are secured by default, keep the reset
values in BL2.

Change-Id: I9e560d936f8e8fda0f96f6299bb0c3b35ba9b71f
Signed-off-by: Yann Gautier <yann.gautier@st.com>