ARM: dts: imx7: Mark timer with arm,cpu-registers-not-fw-configured

There is no firmware to initialize the arch timer register on all iMX7
systems, add arm,cpu-registers-not-fw-configured to let Linux do it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>