tcl/target/stm32f7x: configure faster system clock in reset-init

tcl/target/stm32f7x: configure faster system clock in reset-init

STM32F7xx devices need faster clock for flash programming
over JTAG transport. Using reset default 16 MHz clock
resulted in lot of DAP WAITs and substantial decrease
of flashing performance.

Adapted to the restructured dap support
(see 2231da8ec4e7d7ae9b652f3dd1a7104f5a110f3f).

Change-Id: Ida6915331dd924c9c0d08822fd94c04ad408cdc5
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4464
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>