optee: Refuse to load the driver under the kdump kernel

Fix a hung task issue, seen when booting the kdump kernel, that is
caused by all of the secure world threads being in a permanent suspended
state:

 INFO: task swapper/0:1 blocked for more than 120 seconds.
       Not tainted 5.4.83 #1
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 swapper/0       D    0     1      0 0x00000028
 Call trace:
  __switch_to+0xc8/0x118
  __schedule+0x2e0/0x700
  schedule+0x38/0xb8
  schedule_timeout+0x258/0x388
  wait_for_completion+0x16c/0x4b8
  optee_cq_wait_for_completion+0x28/0xa8
  optee_disable_shm_cache+0xb8/0xf8
  optee_probe+0x560/0x61c
  platform_drv_probe+0x58/0xa8
  really_probe+0xe0/0x338
  driver_probe_device+0x5c/0xf0
  device_driver_attach+0x74/0x80
  __driver_attach+0x64/0xe0
  bus_for_each_dev+0x84/0xd8
  driver_attach+0x30/0x40
  bus_add_driver+0x188/0x1e8
  driver_register+0x64/0x110
  __platform_driver_register+0x54/0x60
  optee_driver_init+0x20/0x28
  do_one_initcall+0x54/0x24c
  kernel_init_freeable+0x1e8/0x2c0
  kernel_init+0x18/0x118
  ret_from_fork+0x10/0x18

The invoke_fn hook returned OPTEE_SMC_RETURN_ETHREAD_LIMIT, indicating
that the secure world threads were all in a suspended state at the time
of the kernel crash. This intermittently prevented the kdump kernel from
booting, resulting in a failure to collect the kernel dump.

Make kernel dump collection more reliable on systems utilizing OP-TEE by
refusing to load the driver under the kdump kernel.

Cc: stable@vger.kernel.org
Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>