Mbed TLS: Remove weak heap implementation

The implementation of the heap function plat_get_mbedtls_heap() becomes
mandatory for platforms supporting TRUSTED_BOARD_BOOT.

The shared Mbed TLS heap default weak function implementation is
converted to a helper function get_mbedtls_heap_helper() which can be
used by the platforms for their own function implementation.

Change-Id: Ic8f2994e25e3d9fcd371a21ac459fdcafe07433e
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>