arm64: dts: ls1028a: Update flexcan properties

LS1028A supports two flexcan controllers similar to LX2160A.

There's already a compatible entry defined i.e "fsl,lx2160ar1-flexcan"
which can be further reused for LS1028A.
Please note, "fsl,ls1028ar1-flexcan" compatible entry doesn't exists and
can be safely removed.

LS1028A has a single peripheral clock (i.e platform clock) source
connected to both "ipg" and "per" and therefore, remove "sysclk" as
clock source from device-tree.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>