ARM: dts: imx7d: add arch timer

Since uboot v2016.01-rc2, which supported basic psci for i.mx7d.
So imx7d's second core can be enabled by psci.

Without arch timer, every timer event will be boardcasted to each core.
arch timer has local timer irq for each core.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>