fconf: Add mbedtls shared heap as property

fconf: Add mbedtls shared heap as property

Use the firmware configuration framework in arm dynamic configuration
to retrieve mbedtls heap information between bl1 and bl2.

For this, a new fconf getter is added to expose the device tree base
address and size.

Change-Id: Ifa5ac9366ae100e2cdd1f4c8e85fc591b170f4b6
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>