sifive: fu540: Add sample SD gpt partition layout

sifive: fu540: Add sample SD gpt partition layout

This is a sample GPT partition layout for SD card,
right now three important partitions are added to
make the system bootable.

partition layout:

Part    Start LBA       End LBA         Name
        Attributes
        Type GUID
        Partition GUID
  1     0x00000022      0x00000821      "loader1"
        attrs:  0x0000000000000000
        type:   5b193300-fc78-40cd-8002-e86c45580b47
        guid:   cbcbef44-e627-42bc-b134-93b6f3784b8c
  2     0x00000822      0x00002821      "loader2"
        attrs:  0x0000000000000000
        type:   2e54b353-1271-4842-806f-e436d6af6985
        guid:   f54eba28-d8de-4852-978d-1a673777e2ae
  3     0x00002822      0x00020821      "rootfs"
        attrs:  0x0000000000000004
        type:   0fc63daf-8483-4772-8e79-3d69d8477de4
        type:   linux
        guid:   9561df46-8d55-4799-a83b-cfee9ef6ff93

Note:
- loader1 would be fsbl or spl
- loader2 would be U-Boot or U-Boot proper

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>