arm64: dts: ls1028a: fix node name for the sysclk

U-Boot attempts to fix up the "clock-frequency" property of the "/sysclk" node:
https://elixir.bootlin.com/u-boot/v2021.04/source/arch/arm/cpu/armv8/fsl-layerscape/fdt.c#L512

but fails to do so:

  ## Booting kernel from Legacy Image at a1000000 ...
     Image Name:
     Created:      2021-06-08  10:31:38 UTC
     Image Type:   AArch64 Linux Kernel Image (gzip compressed)
     Data Size:    15431370 Bytes = 14.7 MiB
     Load Address: 80080000
     Entry Point:  80080000
     Verifying Checksum ... OK
  ## Flattened Device Tree blob at a0000000
     Booting using the fdt blob at 0xa0000000
     Uncompressing Kernel Image
     Loading Device Tree to 00000000fbb19000, end 00000000fbb22717 ... OK
  Unable to update property /sysclk:clock-frequency, err=FDT_ERR_NOTFOUND

  Starting kernel ...

All Layerscape SoCs except LS1028A use "sysclk" as the node name, and
not "clock-sysclk". So change the node name of LS1028A accordingly.

Fixes: 8897f325

 ("arm64: dts: Add support for NXP LS1028A SoC")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>