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: NPeter Robinson <pbrobinson@gmail.com>
Reviewed-by: NKever Yang <kever.yang@rock-chips.com>

configs/rock960-rk3399_defconfig

@@ -23,6 +23,7 @@ CONFIG_CMD_MMC=y
 CONFIG_CMD_PCI=y
 CONFIG_CMD_USB=y
 # CONFIG_CMD_SETEXPR is not set
+# CONFIG_CMD_SF is not set
 CONFIG_CMD_TIME=y
 CONFIG_CMD_PMIC=y
 CONFIG_CMD_REGULATOR=y