ARM: dts: msm: add discard flag on system mount

Discard flag helps to resolve adb reboot problem.
Sometime after adb reboot device enter into fastboot
mode because system is not mount.

Change-Id: I53c153a3d93c4cba6b54570ba9725789eca28964
Signed-off-by: Pranshu Gupta <pransh@codeaurora.org>