arm64: dts: mediatek: mt8186: Fix systimer 13 MHz clock description

arm64: dts: mediatek: mt8186: Fix systimer 13 MHz clock description

The systimer block derives its 13 MHz clock by dividing the main 26 MHz
oscillator clock by 2 internally. The 13 MHz clock is not a separate
oscillator.

Fix this by making the 13 MHz clock a divide-by-2 fixed factor clock,
taking its input from the main 26 MHz oscillator.

Fixes: 2e78620b1350 ("arm64: dts: Add MediaTek MT8186 dts and evaluation board and Makefile")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20221201084229.3464449-5-wenst@chromium.org
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>