riscv: set fdtfile on VisionFive 2

Multiple revisions of the StarFive VisionFive 2 board exist. They can be
identified by reading their EEPROM.

Linux uses two differently named device-tree files. To load the correct
device-tree we need to set $fdtfile to the device-tree file name that
matches the board revision.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>