clk: add clk interface with clk_ops register

Introduce a minimal clock framework.

Change-Id: I5119a2aeaf103ceaae7a60d9e423caf0c148d794
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/182345


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>
Tested-by: Yann GAUTIER <yann.gautier@st.com>