STM32H7: Support the RNG Peripheral

The random number generator on the STM32H7 is identical to the v1 RNG
already present in the library. So use that, and make sure that the RCC
knows about the peripheral.

Originally filed at: https://github.com/libopencm3/libopencm3/pull/1244

Reviewed-by: Karl Palsson <karlp@tweak.net.au>
 * whitespace changes from review

lib/stm32/h7/Makefile

@@ -44,6 +44,7 @@ OBJS += fmc_common_f47.o
 OBJS += gpio_common_all.o gpio_common_f0234.o
 OBJS += pwr.o rcc.o
 OBJS += rcc_common_all.o
+OBJS += rng_common_v1.o
 OBJS += spi_common_all.o spi_common_v2.o
 OBJS += timer_common_all.o
 OBJS += usart_common_v2.o usart_common_fifos.o