ARM: dts: imx6sx-sdb: Fix the reg properties for the FSL QSPI nodes

The current driver does not use the reg properties, but we will
add a new driver soon. To make sure we have a consistent scheme,
let's fix the reg properties here.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>