ENGR00209059-1 MX6: reboot: add reboot to special function

ENGR00209059-1 MX6: reboot: add reboot to special function

add reboot to special function like mfg download mode,
android fastboot, recovery mode.

It use ASRC register to enter mfgtool download mode and other function.
For android fastboot, recovery function it use ASRC_GPR10 bit 7-8 bit,
it will checked in uboot and clear after read.

Add this feature to improve recovery function, to avoid infinit looping
enter recovery mode if some thing goes wrong in fastboot mode.
Also add convient function for developer.

usage:

download mode: "reboot download"
fastboot     : "reboot fastboot"
recovery mode: "reboot recovery"

Signed-off-by: Zhang Jiejing <jiejing.zhang@freescale.com>