boards/nucleo-l476: enabled hwrng feature

- enabled hwrng feature
- fixed 48MHz clock selection for the STM32L4

boards/nucleo-l476/Makefile.features

@@ -1,6 +1,7 @@
 # Put defined MCU peripherals here (in alphabetical order)
 FEATURES_PROVIDED += periph_cpuid
 FEATURES_PROVIDED += periph_gpio
+FEATURES_PROVIDED += periph_hwrng
 FEATURES_PROVIDED += periph_pwm
 FEATURES_PROVIDED += periph_spi
 FEATURES_PROVIDED += periph_timer

cpu/stm32l4/cpu.c

@@ -143,6 +143,9 @@ static void cpu_clock_init(void)
     while (!(RCC->CSR & RCC_CSR_LSIRDY)) {}
 #endif
 
+    /* select the MSI clock for the 48MHz clock tree (USB, RNG) */
+    RCC->CCIPR = (RCC_CCIPR_CLK48SEL_0 | RCC_CCIPR_CLK48SEL_1);
+
     /* if configured: enable the HSE clock */
 #if CLOCK_HSE
     RCC->CR |= RCC_CR_HSEON;