arm: board: Fix stm32f4_disco defconfig

This change fixes the prescaler values in the defconfig. The prescaler
values shown in the technical reference manual (STM32F407xx, p.19)
differ from those in the defconfig which results in wrong timer delays.
This patch corrects those values according to the manual. I tested it by
toggling a GPIO pin with different timer frequencies and run the
sanitycheck.

Signed-off-by: Andreas Kölbl <andreas.koelbl@st.oth-regensburg.de>