Support call same level initcall async in kthread.
Introduce kernel parameter "initcall_nr_threads" control how many threads.
initcall_nr_threads default is 0, which disable intcall async.
initcall_nr_threads=-1, auto selected the number of threads.
Save boot time about 29ms on rk3126-bnd-d708 board.
Change-Id: I04663d22500bc1d65ca5841fb08e4b57083d6b5e
Signed-off-by: Tao Huang <huangtao@rock-chips.com>