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Updated the documentation with latest Mbed TLS supported
version i.e. Mbed TLS v2.26.0

Fixes available in this version of Mbed TLS mainly affect
key generation/writing and certificates writing, which
are features used in the cert_create tool.

Release notes of Mbed TLSv2.26.0 are available here:
https://github.com/ARMmbed/mbedtls/releases/tag/v2.26.0



Change-Id: Ie15ee45d878b7681e15ec4bf64d54b416a31aa2f
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>