dm: avoid NULL dereference in add_item()

acpi_add_other_item() passes dev = NULL. Instead of dev->name write the
string "other" to the debug log:

    ACPI: Writing ACPI tables at 1fd3000
    0base: writing table '<NULL>'
    * other: Added type 3, 0000000011fd4000, size 240
    1facs: writing table 'FACS'
    * other: Added type 3, 0000000011fd4240, size 40
    5csrt: writing table 'CSRT'
    * other: Added type 3, 0000000011fd4280, size 30

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>