arm64: dts: add imx8mp-libra-rdk-fpsc board

arm64: dts: add imx8mp-libra-rdk-fpsc board

Add device tree for the Libra-i.MX 8M Plus FPSC board. The Libra is a
pure development board and has hardware to support FPSC-24-A.0 set of
features. It can be populated with the phyCORE-i.MX 8M Plus SoM to form
a SBC.
The phyCORE-i.MX 8M Plus FPSC [1] SoM uses only a subset of the hardware
features the Libra board provides. The phyCORE-i.MX8MP FPSC itself is a
System on Module based on the i.MX 8M Plus SoC utilizing the Future
Proof Solder Core [2] standard.

To be able to easily map FPSC interface names to SoC interfaces, the
FPSC interface names are added as inline comments. Example:

&i2c5 { /* FPSC I2C4 */
	pinctrl-0 = <&pinctrl_i2c5>;
	[...]
};

Here, I2C4 is the FPSC interface name. The i2c5 instance of the i.MX 8M Plus
SoC is used to fulfill the i2c functionality and its signals are routed
to the FPSC I2C4 signal pins:

pinctrl_i2c5: i2c5grp {
	fsl,pins = <
		MX8MP_IOMUXC_SPDIF_RX__I2C5_SDA		0x400001c2	/* I2C4_SDA */
		MX8MP_IOMUXC_SAI5_RXD0__I2C5_SCL	0x400001c2	/* I2C4_SCL */
	>;
};

The features are almost identical to the existing phyCORE-i.MX 8M Plus
SoM (dts: imx8mp-phycore-som.dtsi), but the pin muxing is different due
to the FPSC standard as well as 1.8V IO voltage instead of 3.3V.

[1] https://www.phytec.eu/en/produkte/system-on-modules/phycore-imx-8m-plus-fpsc/
[2] https://www.phytec.eu/en/produkte/system-on-modules/fpsc/

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>