sifive: fu540: Mark the default env as SPI flash

Mark the default U-Boot environment as SPI flash since
this is an on board flash device.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>