rockchip: rk3399: rock960: explicitly disable SPI flash

The Rock960 doesn't have SPI flash on-board, but the bits
get enabled by default which means when booting we get
some errors. Explicitly disable it to stop the errors.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>