ARM: dts: armada388-clearfog: move SPI flash into microsom

The optional SPI flash is fitted to the microsom, not the clearfog
board, so it should be specified in the microsom DTS include file.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>