ARM: dts: am57xx-idk-common: Enable support for QSPI

AM572x IDK has a Spansion s25fl256s1 QSPI flash on the EVM connected to
TI QSPI IP over CS0. Hence, add QSPI and flash slave DT nodes.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>