ARM: dts: imx7: Fix Toradex Colibri iMX7S 256MB NAND flash support

Turns out when introducing the eMMC version the gpmi node required for
NAND flash support got enabled exclusively on Colibri iMX7D 512MB.

Fixes: f928a4a3 ("ARM: dts: imx7: add Toradex Colibri iMX7D 1GB (eMMC) support")
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>