Additional runtime check for DTB presence in BL2

Additional runtime check for DTB presence in BL2

In Mbed TLS shared heap code, an additional sanity check is introduced
in BL2. Currently, when BL2 shares heap with BL1, it expects the heap
info to be found in the DTB. If for any reason the DTB is missing, BL2
cannot have the heap address and, hence, Mbed TLS cannot proceed. So,
BL2 cannot continue executing and it will eventually crash.  With this
change we ensure that if the DTB is missing BL2 will panic() instead of
having an unpredictable crash.

Change-Id: I3045ae43e54b7fe53f23e7c2d4d00e3477b6a446
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>