arm64: dts: mediatek: Correct system timer clock of MT8192

When the initial devicetree for mt8192 was added in 48489980 ("arm64:
dts: Add Mediatek SoC MT8192 and evaluation board dts and Makefile"), the
clock driver for mt8192 was not yet upstream, so the clock property nodes
were set to the clk26m clock as a placeholder.

Given that the clock driver has since been added through 710573de ("clk:
mediatek: Add MT8192 basic clocks support"), as well as its dt-bindings
through f35f1a23 ("clk: mediatek: Add dt-bindings of MT8192 clocks") and
devicetree nodes through 5d2b897b ("arm64: dts: mediatek: Add mt8192
clock controllers"), fix the systimer clock property to point to the actual
clock.

Signed-off-by: Allen-KH Cheng <Allen-KH.Cheng@mediatek.com>
Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220113065822.11809-6-allen-kh.cheng@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>