async: Add cmdline option to specify drivers to be async probed

synchronous driver probing can help much on kernel fastboot, and
this option can provide a flexible way to optimize and quickly verify
async driver probe.

Also it will help in below cases:
* Some driver actually covers several families of HWs, some of which
  could use async probing while others don't. So we can't simply
  turn on the PROBE_PREFER_ASYNCHRONOUS flag in driver, but use this
  cmdline option, like igb driver async patch discussed at
  https://www.spinics.net/lists/netdev/msg545986.html



* For SOC (System on Chip) with multiple spi or i2c controllers, most
  of the slave spi/i2c devices will be assigned with fixed controller
  number, while async probing may make those controllers get different
  index for each boot, which prevents those controller drivers to be
  async probed. For platforms not using these spi/i2c slave devices,
  they can use this cmdline option to benefit from the async probing.

Suggested-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Change-Id: I1fd8d6f87819bd877de9ac19daf3d009a1362b90
Git-repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Git-commit: 1ea61b68d0f8685775c897c2de040c73b8d1c56a
Signed-off-by: Lei wang <leiwan@codeaurora.org>