arm64: dts: imx8mp: Add optional nominal drive mode DTSI

Unlike the i.MX8MM and i.MX8MN SoCs added earlier, the device tree for
the i.MX8MP configures some clocks at frequencies that are only validated
for overdrive mode, i.e. when VDD_SOC is 950 mV.

Boards may want to run their SoC at the lower voltage of 850 mV though
to reduce heat generation and power usage. For this to work, clock rates
need to adhere to the limits of the nominal drive mode.

Add an optional DTSI file which can be included by various boards to run
in this mode.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>