arm64: dts: imx8mp: Add missing speed grade phandle

And missing speed grade phandle to cpu@0 node.

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