powerpc/powernv/prd: Unregister OPAL_MSG_PRD2 notifier during module unload

Commit 587164cd, introduced new opal message type (OPAL_MSG_PRD2) and
added opal notifier. But I missed to unregister the notifier during
module unload path. This results in below call trace if you try to
unload and load opal_prd module.

Also add new notifier_block for OPAL_MSG_PRD2 message.

Sample calltrace (modprobe -r opal_prd; modprobe opal_prd)
  BUG: Unable to handle kernel data access on read at 0xc0080000192200e0
  Faulting instruction address: 0xc00000000018d1cc
  Oops: Kernel access of bad area, sig: 11 [#1]
  LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA PowerNV
  CPU: 66 PID: 7446 Comm: modprobe Kdump: loaded Tainted: G            E     5.14.0prd #759
  NIP:  c00000000018d1cc LR: c00000000018d2a8 CTR: c0000000000cde10
  REGS: c0000003c4c0f0a0 TRAP: 0300   Tainted: G            E      (5.14.0prd)
  MSR:  9000000002009033 <SF,HV,VEC,EE,ME,IR,DR,RI,LE>  CR: 24224824  XER: 20040000
  CFAR: c00000000018d2a4 DAR: c0080000192200e0 DSISR: 40000000 IRQMASK: 1
  ...
  NIP notifier_chain_register+0x2c/0xc0
  LR  atomic_notifier_chain_register+0x48/0x80
  Call Trace:
    0xc000000002090610 (unreliable)
    atomic_notifier_chain_register+0x58/0x80
    opal_message_notifier_register+0x7c/0x1e0
    opal_prd_probe+0x84/0x150 [opal_prd]
    platform_probe+0x78/0x130
    really_probe+0x110/0x5d0
    __driver_probe_device+0x17c/0x230
    driver_probe_device+0x60/0x130
    __driver_attach+0xfc/0x220
    bus_for_each_dev+0xa8/0x130
    driver_attach+0x34/0x50
    bus_add_driver+0x1b0/0x300
    driver_register+0x98/0x1a0
    __platform_driver_register+0x38/0x50
    opal_prd_driver_init+0x34/0x50 [opal_prd]
    do_one_initcall+0x60/0x2d0
    do_init_module+0x7c/0x320
    load_module+0x3394/0x3650
    __do_sys_finit_module+0xd4/0x160
    system_call_exception+0x140/0x290
    system_call_common+0xf4/0x258

Fixes: 587164cd ("powerpc/powernv: Add new opal message type")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211028165716.41300-1-hegdevasant@linux.vnet.ibm.com