hw/riscv: Make FDT optional for MPFS

hw/riscv: Make FDT optional for MPFS

Real-time kernels such as RTEMS or Zephyr may use a static device tree
built into the kernel image.  Do not require to use the -dtb option if
-kernel is used for the microchip-icicle-kit machine.  Issue a warning
if no device tree is provided by the user since the machine does not
generate one.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250319061342.26435-4-sebastian.huber@embedded-brains.de>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>