mbedtls: Define optimized mbed TLS heap size

mbedtls: Define optimized mbed TLS heap size

mbed TLS provides the debug API `mbedtls_memory_buffer_alloc_status()`
to analyse the RAM usage of the library.

When RSA is selected as algorithm, the maximum heap usage in FVP and
Juno has been determined empirically to be approximately 5.5 KiB.
However, The default heap size used when RSA is selected is 8 KiB.

This patch reduces the buffer from 8 KiB to 6 KiB so that the BSS
sections of both BL1 and BL2 are 2 KiB smaller when the firmware is
compiled with TBB support.

Change-Id: I43878a4e7af50c97be9c8d027c728c8483f24fbf
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>