arm64: dts: ls1028a: correct the lane mapping property for mhdp driver

This issue is introduced by commit 6d93b9ba("arm64: dts:
ls1028a: enable DP node for RDB/QDS boards"), it uses the wrong
string as the lane mapping property. So, correct the property string
to "lane-mapping" to keep consistent with mhdp driver code.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>