fix(stm32mp1): fix missing padding to verify signature

ECDSA signatures can not guarantee the exact 32 bits that must
be given to the hardware. We must check the ASN1 sequence and
adapt the R,S tuple to manage a proper 2 * 32 bytes whatever
the certificate content.

Signed-off-by: Lionel Debieve <lionel.debieve@foss.st.com>
Change-Id: I456c32aa1d081562c2787866894f3be3b6f8cb9d
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/252923


Reviewed-by: CITOOLS <MDG-smet-aci-reviews@list.st.com>