ARM: imx: remove imx_src_prepare_restart() call

There is ~10% possibility that the following emergency restart command
fails to reboot imx6q.

$ echo b > /proc/sysrq-trigger

The IMX restart routine mxc_restart() assumes that it will always run on
primary core, and will call imx_src_prepare_restart() to disable
secondary cores in order to get them come to online in the following
boot.  However, the assumption is only true for normal kernel_restart()
case where migrate_to_reboot_cpu() will be called to migrate to primary
core, but not necessarily true for emergency_restart() case.  So when
emergency_restart() calls into mxc_restart() on any secondary core,
system will hang immediately once imx_src_prepare_restart() is called
to disabled secondary cores.  Since emergency_restart() is defined as a
function that is safe to call in interrupt context, we cannot just call
migrate_to_reboot_cpu() to fix the issue.

Fortunately, we just found that the issue can be fixed at imx6q platform
level.  We used to call imx_src_prepare_restart() to disable all
secondary cores before resetting hardware.  Otherwise, the secondary
will fail come to online in the reboot.  However, we recently found that
after commit 6050d181 (ARM: imx: reset core along with enable/disable
operation) comes to play, we do not need to reset the secondary cores
any more.  That said, mxc_restart() now can run on any core to reboot
the system, as long as we remove the imx_src_prepare_restart() call from
mxc_restart().

So let's simply remove imx_src_prepare_restart() call to fix the above
emergency restart failure.

Reported-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>