ARM: dts: imx7: add QSPI

In preparation for an update of the TQ-Systems TQMa7x/MBa7x DTS, add the
QSPI controller to imx7s.dtsi.

Based-on-patch-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>