board: Add FriendlyARM Nanopi A64 support

Add initial support for FriendlyARM Nanopi A64
board with below features:
- U-Boot 2017.11
- Linux 4.14
- Default packages from buildroot

Cc: Yuefei <yftan@friendlyarm.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/friendlyarm/nanopi-a64/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-a64-nanopi-a64.dtb
+
+booti $kernel_addr_r - $fdt_addr_r

board/friendlyarm/nanopi-a64/genimage.cfg

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+image boot.vfat {
+	vfat {
+		files = {
+			"Image",
+			"sun50i-a64-nanopi-a64.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/friendlyarm/nanopi-a64/readme.txt

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+Intro
+=====
+
+This default configuration will allow you to start experimenting with the
+buildroot environment for the Nanopi A64. With the current configuration
+it will bring-up the board, and allow access through the serial console.
+
+Nanopi A64 link:
+http://nanopi.io/nanopi-a64.html
+
+Wiki link:
+https://openedev.amarulasolutions.com/display/ODWIKI/FriendlyARM+Nanopi+A64
+
+This configuration uses U-Boot mainline and kernel mainline.
+
+How to build
+============
+
+    $ make friendlyarm_nanopi_a64_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 Nanopi A64 and power it up. The console
+is on the serial line, 115200 8N1.