ARM: dts: imx6ul: segin: Only enable NAND if it is populated

The phyCORE-i.MX 6UL/ULL now comes either with NAND flash or eMMC. We
have to configure the populated memory type in the device tree files. So
the GPMI node gets disabled by default and only enabled if populated.

Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>