mkimage: Set correct FDT type and ramdisk architecture in FIT auto mode

mkimage: Set correct FDT type and ramdisk architecture in FIT auto mode

When running the following command

    mkimage -f auto -A arm -O linux -T kernel -C none -a 0x8000 -e 0x8000 \
            -d zImage -b zynq-microzed.dtb -i initramfs.cpio image.ub

the type of fdt subimage is the same as of the main kernel image and
the architecture of the initramfs image is not set. Such an image is
refused by U-Boot when booting. This commits sets the mentioned
attributes, allowing to use the "-f auto" mode in this case instead of
writing full .its file.

Following is the diff of mkimage output without and with this commit:

 FIT description: Kernel Image image with one or more FDT blobs
 Created:         Thu Sep 12 23:23:16 2019
  Image 0 (kernel-1)
   Description:
   Created:      Thu Sep 12 23:23:16 2019
   Type:         Kernel Image
   Compression:  uncompressed
   Data Size:    4192744 Bytes = 4094.48 KiB = 4.00 MiB
   Architecture: ARM
   OS:           Linux
   Load Address: 0x00008000
   Entry Point:  0x00008000
  Image 1 (fdt-1)
   Description:  zynq-microzed
   Created:      Thu Sep 12 23:23:16 2019
-  Type:         Kernel Image
+  Type:         Flat Device Tree
   Compression:  uncompressed
   Data Size:    9398 Bytes = 9.18 KiB = 0.01 MiB
   Architecture: ARM
-  OS:           Unknown OS
-  Load Address: unavailable
-  Entry Point:  unavailable
  Image 2 (ramdisk-1)
   Description:  unavailable
   Created:      Thu Sep 12 23:23:16 2019
   Type:         RAMDisk Image
   Compression:  Unknown Compression
   Data Size:    760672 Bytes = 742.84 KiB = 0.73 MiB
-  Architecture: Unknown Architecture
+  Architecture: ARM
   OS:           Linux
   Load Address: unavailable
   Entry Point:  unavailable
  Default Configuration: 'conf-1'
  Configuration 0 (conf-1)
   Description:  zynq-microzed
   Kernel:       kernel-1
   Init Ramdisk: ramdisk-1
   FDT:          fdt-1
   Loadables:    kernel-1

Signed-off-by: Michal Sojka <michal.sojka@cvut.cz>