Remove Mbed TLS dependency from plat_bl_common.c

Due to the shared Mbed TLS heap optimisation introduced in 6d01a463,
common code files were depending on Mbed TLS specific headers. This
dependency is now removed by moving the default, unoptimised heap
implementation inside the Mbed TLS specific files.

Change-Id: I11ea3eb4474f0d9b6cb79a2afd73a51a4a9b8994
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>