drivers/usb_dc_stm32: HSI48 requires VREFINT in L0

In STM32L0 series, HSI48 requires VREFINT and its buffer
with 48 MHz RC to be enabled. This patch enables
VREFINT reference for HSI48 oscillator.

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