ARM: dts: am57xx: cl-som-am57x: add spi-flash support

On-board spi-flash chip is used as a main boot device.
Add spi-flash chip support (over QSPI bus).

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>