mtd: brcmnand: drop brcmnand_host::of_node field

We don't actually need to stash a copy of this device_node indefinitely;
we only need it in brcmnand_init_cs().

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: <bcm-kernel-feedback-list@broadcom.com>
Cc: Kamal Dasu <kdasu.kdev@gmail.com>
Acked-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>