arm/fdt.c: Don't generate the node if generator function is NULL

Print a more helpful debugging message when a MMIO device hasn't set a
function to generate an FDT node instead of causing a segmentation fault by
dereferencing a NULL pointer.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>