sifive: fu540: Add Booting from SPI

Add booting from SPI for SiFive Unleashed board.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
3 files changed