Video overlays for C72 Nano can also be used for C72 Mini

The internal watchdog was interfering with the external APX watchdog.

Use the alias rtc0 to force pcf2123 as the primary RTC for the system,
while relegating the internal snvs_rtc as secondary (rtc1)

USB interfaces management on C72 Mini:
 - USB1 is OTG
 - USB2 is HOST and connected to the USB HUB

Clocks are specific for I.MX8M Mini SoC

- Import C72 common include file
- Add C72 Mini pinmuxing from kernel 5.4
(https://git.seco.com/edgehog/bsp/nxp/linux-seco-imx/-/blob/seco/develop/imx_5.4.70_2.3.0/arch/arm64/boot/dts/seco/seco-imx8mm-c72.dts)

- Delete nodes present in the common file
- Import common include file

To support polarity inversion i.MX8MP DT has been changed use 3 pwm-cells.
Align lcd backlights pwm by selecting normal polarity.

The regulators related to `vcc_blk_sw` and `vcc_lcd_sw` have been
modified removing the `regulator-always-on` entry. This change requires
that:
- the backlight node has the `enable-gpio` and the `power-supply`
correctly set (now added);
- the panel node has the `backlight`, `power-supply` and `enable-gpio`
correctly set (now added).

The regulator enabling the backlight has been removed because it has the
same function of `enable-gpio` in the backlight node.

Additionally, all the display-related nodes have been refactored to
move all the related nodes into their overlays.

The watchdog feature is activated by default from the SCFW.

The watchdog node was already configured in the file `imx8qxp.dtsi`
that is included in C57's device tree.

This commit only copies the watchdog node to the linux device tree to
configure the timeout easier.

Tested the panel Innolux G156HCE-L01.

Tested with FN101PGRGUL034C 1280*800 with cable LVDS CV-1412/240.

The panel is AUO G101EAN02.

Tested with FN070PGRGUL037C 800x480 with cable LVDS CV-1408/750.

The panel is Innolux NE070NB-04F.

The I2C1 is connected to the I2C touch socket.

Test with panel BOE EV156FHM-N10 1920x1080.

Requires the firmware to be in the filesystem. For this, it requires
the machine configuration to include:
```
MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS:append = " \
   linux-firmware-sd8997 \
"
```

Add overlay to enable the LVDS panel 1024x600.

Add to the device tree the required nodes to load the file system
from USB 3.0 or eMMC.

Add the minimal device tree in order to correctly boot the board
loading the filesystem with NFS (USB and eMMC are not supported
yet).