ARM: dts: imx6sll: remove unused property in gpc node

The "fsl,mf-mix-wakeup-irq" is ONLY used as a temporary
solution in NXP's internal tree for Mega/Fast Mix off
feature after suspend, upstream kernel does NOT need it,
remove it.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>