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@xxxxxxxxxxxx>