docs(prerequisites): upgrade to Mbed TLS 2.28.1

docs(prerequisites): upgrade to Mbed TLS 2.28.1

In anticpation of the next Trusted Firmware release update the to newest
2.x Mbed TLS library [1].

Note that the Mbed TLS project published version 3.x some time ago.
However, as this is a major release with API breakages, upgrading to
this one might require some more involved changes in TF-A, which we are
not ready to do. We shall upgrade to Mbed TLS 3.x after the v2.8 release
of TF-A.

[1] https://github.com/Mbed-TLS/mbedtls/tree/v2.28.1

Change-Id: I7594ad062a693d2ecc3b1705e944dce2c3c43bb2
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>