bsec: add scratch register access API

Add read/write functions for BSEC scratch register, used for communication
with external agent, and to store boot parameters.

Change-Id: If024bcce127c829d8a663df0119f0e908712405e
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/187860


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