drivers: clock: stm32: overdrive after sysclock

According to the reference manual the overdrive should be enabled after
setup of the sysclock (HSE or HSI) and enabling the PLL (PLLON).
The flash latency should be enabled after the PLL has been turned on,
but before switching the system clock to the PLL.

Signed-off-by: Max van Kessel <max_van_kessel@msn.com>