clock_control/stm32f1: HSE support and PLL configuration cleanup

Add support for use of HSE (incorrectly named PREDIV1)as input of PLL,
along with HSE bypass for stabilized external clock, and XTPRE
prescaler. Update PLL handling so that we do not unnecessarily enable
PLL clocks, instead enabling only the clocks sources that are required
as per user's configuration.

This change allows higher SYSCLK clock values, up to 72MHz.

Change-Id: Ia7c2be3ce11ac0de2efa664b20e7ab5fddd57a51
Origin: Original
Signed-off-by: Maciej Borzecki <maciek.borzecki@gmail.com>