board: rzg2l: Check the DTB pointer passed by the TF-A.

board: rzg2l: Check the DTB pointer passed by the TF-A.

On the RZG2L platform, the advised
TF-A (https://github.com/renesas-rz/rzg_trusted-firmware-a/tree/v2.5/rzg2l)
does not pass any DTB blob to U-Boot.

On the other hand, the RZG2L part of U-Boot expects a DTB to be passed.  It
means that if one flashes the latest TF-A as well as the mainline U-Boot,
it will crash trying to dereference the NULL DTB pointer before outputing
anything.

Check if the DTB pointer is NULL before trying to use it.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>