arm: dts: k3-j72xx: correct MCU timer1 frequency

arm: dts: k3-j72xx: correct MCU timer1 frequency

MCU timer1 is used as the tick timer for MCU R5 SPL, and the
clock-frequency defined in DT appears to be incorrect at the moment.
Actual clock source for the timer is MCU_SYSCLK0 / 4 which is 250MHz.

Earlier setup of 25MHz went unnoticed, as there was a separate issue
with omap-timer, which caused an error to the clock by a factor of 8
with j7 devices. This problem surfaced once the omap-timer was fixed.

Signed-off-by: Tero Kristo <kristo@kernel.org>