drivers: core: nullify gd->dm_root after dm_uninit()

drivers: core: nullify gd->dm_root after dm_uninit()

To reset the DM after a new dtb is loaded, we need to call dm_uninit()
and then dm_init(). This fails however because gd->dm_root is not nullified
by dm_uninit().
Fixing it by setting gd->dm_root in dm_uninit().

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>