tests: drivers: Assign prescaler value to 10000

This commit modifies st prescaler value from 0 to 10000 for
STM32 nucleo_f207zg. So pwm_api test results in successful
execution on nucleo_f207zg board.

Signed-off-by: Sidhdharth Yadav <sidhdharth.yadav@hcl.com>