drivers/clock_control: stm32: Configure and enable PLL2

DT node "st,stm32f105-pll2-clock" already exists but was not actually used
and PLL2 was not being configured.
PLL2 is available on STM32F105/F107 and should be turned off after turning
off PLL and turned on before turning on PLL again since PLL2 can be
used as a source for PLL. Source for PLL2 is always HSE.

Signed-off-by: Pierre-Emmanuel Novac <piernov@piernov.org>