drivers/stm32l0x_ll_clock: Enable SYSCFG in clock_control

Enable System Configuration Controller clock in clock_control
instead of gpio controller when we set a gpio as interrupt.
We use System Configuration Controller to manage external
interrupts and internal voltage reference for 48 MHz HSI
used in USB device.

Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>