drivers/of: Validate device node in __unflatten_device_tree()

@mynodes is set to NULL when __unflatten_device_tree() is called
to unflatten device sub-tree in PCI hot add scenario on PowerPC
PowerNV platform. Marking @mynodes detached unconditionally causes
kernel crash as below backtrace shows:

Unable to handle kernel paging request for data at address 0x00000000
Faulting instruction address: 0xc000000000b26f64
cpu 0x0: Vector: 300 (Data Access) at [c000003fcc7cf740]
    pc: c000000000b26f64: __unflatten_device_tree+0xf4/0x190
    lr: c000000000b26f40: __unflatten_device_tree+0xd0/0x190
    sp: c000003fcc7cf9c0
   msr: 900000000280b033
   dar: 0
 dsisr: 40000000
  current = 0xc000003fcc281680
  paca    = 0xc00000000ff00000	 softe: 0	 irq_happened: 0x01
    pid   = 2724, comm = sh
Linux version 4.7.0-gavin-07754-g92a6836 (gwshan@gwshan) (gcc version \
4.9.3 (Buildroot 2016.02-rc2-00093-g5ea3bce) ) #539 SMP Mon Aug 1 \
12:40:29 AEST 2016
enter ? for help
[c000003fcc7cfa50] c000000000b27060 of_fdt_unflatten_tree+0x60/0x90
[c000003fcc7cfaa0] c0000000004c6288 pnv_php_set_slot_power_state+0x118/0x440
[c000003fcc7cfb80] c0000000004c6a10 pnv_php_enable+0xc0/0x170
[c000003fcc7cfbd0] c0000000004c4d80 power_write_file+0xa0/0x190
[c000003fcc7cfc50] c0000000004be93c pci_slot_attr_store+0x3c/0x60
[c000003fcc7cfc70] c0000000002d3fd4 sysfs_kf_write+0x94/0xc0
[c000003fcc7cfcb0] c0000000002d2c30 kernfs_fop_write+0x180/0x260
[c000003fcc7cfd00] c000000000230fe0 __vfs_write+0x40/0x190
[c000003fcc7cfd90] c000000000232278 vfs_write+0xc8/0x240
[c000003fcc7cfde0] c000000000233d90 SyS_write+0x60/0x110
[c000003fcc7cfe30] c000000000009524 system_call+0x38/0x108

This avoids the kernel crash by marking @mynodes detached only when
@mynodes is dereferencing valid device node in __unflatten_device_tree().

Fixes: 1d1bde55 ("of: fdt: mark unflattened tree as detached")
Reported-by: Meng Li <shlimeng@cn.ibm.com>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Rob Herring <robh@kernel.org>