ttbr: increase PK_DER_LEN size

Public key brainpool ecdsa DER certificate are 92 byte long:
OID for brainpool curve are 1 byte bigger than the one for NIST curve.

Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: Ifad51da3c576d555da9fc519d2df3d9a0e6ed91b
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/184837


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