dm: dump.c: Fix segfault when entry->of_match is NULL

Currently, dm drivers command produces a segfault:
=> dm drivers
Driver                Compatible
--------------------------------
Segmentation fault (core dumped)

This is caused by a NULL pointer dereference of entry->of_match.
Add a check to prevent this.

Signed-off-by: Ovidiu Panait <ovpanait@gmail.com>
Cc: Sean Anderson <seanga2@gmail.com>
Cc: Simon Glass <sjg@chromium.org>