arm64: dts: imx8ulp: add flexspi node

Add flexspi node, flexspi has a special memory region mapped to
0x60000000~0x6fffffff. This region is for AHB usage. So add this region
to SoC ranges.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>