boards: arm: stm32: Adjust maximum speed for STM32F746G disco board

Currently configuration for STM32F746G Discovery board on reset-start
event is inherited from included files (OpenOCD stm32f7x.cfg), but
contrary to the inherited adapter speed on reset-init event, the speed
inherited for the reset-start event is only 2000 kHz, which is not
available, so a lower speed is picked up automatically generating the
following message several times when flashing the board:

Info : Unable to match requested speed 2000 kHz, using 1800 kHz

That commit overrides that suboptimal speed for reset-start event and
sets it to the same speed as used by reset-init event, i.e. the maximum
speed (4000 kHz), so the noisy messages like the above one disappear.

The change also improves a bit the throughput when writing to the board.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>