ARM: dts: BCM63xx: re-parent NAND controller node

The NAND controller is a child node of the UBUS (legacy) bus, not the
AXI (new) bus, re-parent the NAND controller node accordingly. This was
a mistake introduced by a failed merge of this NAND node with other
changes (PMB).

Fixes: b5762cac ("ARM: bcm63138: add NAND DT support")
Reported-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>