arm64: dts: imx8mn: Add LPDDR4 EVK board support

i.MX8MN LPDDR4 EVK board shares most of the device as DDR4 EVK board,
the ONLY difference are the DDR type and PMIC, add support for it
and make it default i.MX8MN EVK board as usual.

The PMIC driver is NOT ready, so cpu-freq needs to be disabled as
it depends on regulator provided by PMIC.

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