mbedtls: Use `tf_snprintf` instead of `snprintf`

The Trusted Firmware uses a subset of the APIs provided by mbed TLS.
This subset internally uses `snprintf`, but the only format specifier
used is '%d', which is supported by `tf_snprintf`.

This patch makes mbed TLS use `tf_snprintf` instead of `snprintf`,
saving 3 KB in both debug and release builds when TBBR is enabled.

Change-Id: I7f992a21015930d7c0f4660e7a28ceefd60b9597
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>