board: add Orangepi Zero Plus support

Add initial support for Orangepi Zero Plus board:
- U-Boot 2020.10
- Linux 5.7.19
- Default packages from buildroot

This patch is based on Orangepi Zero Plus2 board support, so genimage
configuration and description is copy pasted. One difference is that
newer Linux and U-Boot are selected. Another difference is Ethernet
support (Plus) instead of WiFi (Plus2).

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/orangepi/orangepi-zero-plus/boot.cmd

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+setenv bootargs console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p2 rootwait
+
+fatload mmc 0 $kernel_addr_r Image
+fatload mmc 0 $fdt_addr_r sun50i-h5-orangepi-zero-plus.dtb
+
+booti $kernel_addr_r - $fdt_addr_r

board/orangepi/orangepi-zero-plus/genimage.cfg

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+image boot.vfat {
+	vfat {
+		files = {
+			"Image",
+			"sun50i-h5-orangepi-zero-plus.dtb",
+			"boot.scr"
+		}
+	}
+	size = 64M
+}
+
+image sdcard.img {
+	hdimage {
+	}
+
+	partition spl {
+		in-partition-table = "no"
+		image = "sunxi-spl.bin"
+		offset = 8192
+	}
+
+	partition u-boot {
+		in-partition-table = "no"
+		image = "u-boot.itb"
+		offset = 40K
+		size = 1M # 1MB - 40K
+	}
+
+	partition boot {
+		partition-type = 0xC
+		bootable = "true"
+		image = "boot.vfat"
+	}
+
+	partition rootfs {
+		partition-type = 0x83
+		image = "rootfs.ext4"
+	}
+}

board/orangepi/orangepi-zero-plus/readme.txt

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+Intro
+=====
+
+This default configuration will allow you to start experimenting with the
+buildroot environment for the Orangepi Zero Plus. With the current configuration
+it will bring-up the board, and allow access through the serial console.
+
+Orangepi Zero Plus link:
+http://www.orangepi.org/OrangePiZeroPlus/
+
+This configuration uses U-Boot mainline and kernel mainline.
+
+How to build
+============
+
+    $ make orangepi_zero_plus_defconfig
+    $ make
+
+Note: you will need access to the internet to download the required
+sources.
+
+How to write the SD card
+========================
+
+Once the build process is finished you will have an image called "sdcard.img"
+in the output/images/ directory.
+
+Copy the bootable "sdcard.img" onto an SD card with "dd":
+
+  $ sudo dd if=output/images/sdcard.img of=/dev/sdX
+  $ sudo sync
+
+Insert the micro SDcard in your Orangepi Zero Plus and power it up. The console
+is on the serial line, 115200 8N1.