arm64: dts: khadas-vim3: add SPIFC controller node

Add disabled SPIFC controller node with instruction on how to enable
it while lowering capabilities of the eMMC controller from 8bits bus
width to 4bits bus width, it's data pins 4 to 7 being shared with
the SPI NOR controller pins.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200313090713.15147-4-narmstrong@baylibre.com