periph/hwrng: use void* buf for hwrng_read()

cpu/cc2538/periph/hwrng.c

cpu/kinetis_common/periph/hwrng_rnga.c

cpu/kinetis_common/periph/hwrng_rngb.c

cpu/native/periph/hwrng.c

cpu/nrf5x_common/periph/hwrng.c

cpu/sam3/periph/hwrng.c

cpu/stm32f2/periph/hwrng.c

cpu/stm32f4/periph/hwrng.c

drivers/include/periph/hwrng.h