fix bug for RDA UNO_91H：calling us ticker functions without init.

solution: init us ticker when init TRNG.

Author: caixue1102

targets/TARGET_RDA/TARGET_UNO_91H/trng_api.c

@@ -70,6 +70,8 @@ void trng_init(trng_t *obj)
     regval = rTRNG_CTRL |  ((0x01UL << 4) | (0xFFUL << 16));
     rTRNG_CTRL = regval & ~((0x01UL << 1) | (0x01UL << 2) | (0x01UL << 3));
 
+    us_ticker_init();
+
     /*Entropy data was mixed by TRNG seed and noise, so we add one 32us delay to
       ensure all 32 bits of seed is entropy when init and 
       another delay to update noise data when get data.