feat(st-clock): define secure and non-secure gate clocks

Array stm32mp1_clk_gate[] defines the clock resources. This change
add a secure attribute to the clock: secure upon RCC[TZEN] (SEC),
secure upon RCC[TZEN] and RCC[MCKPROT] (MKP) or always accessible
from non-secure (N_S).

At init, lookup clock tree to check if any of the secure clocks
is derived from PLL3 in which case PLL3 shall be secure.

Note that this change does not grow byte size of stm32mp1_clk_gate[].

Change-Id: I933d8a30007f3c72f755aa1ef6d7e6bcfabbfa9e
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>