uboot-imx

ARM: stm32f7: fix prescaler calculation of timer

As the timer 2 is on APB1 bus, the maximum of clock frequency of APB1 timer
clock is half of SYSCLK. Then to calculate the timer prescaler for timer 2
which need to be divided by 2.

Signed-off-by: Bo Shen <voice.shen@gmail.com>
Reviewed-by: Vikas Manocha <vikas.manocha@st.com>