[cc1352] limit DRBG to only use 128-bit AES keys (#4032)

The CC2650 only has hardware acceleration for 128-bit AES keys. The
CC1352 and CC2652 have hardware acceleration for 128- and 256-bit keys.
However, the current driver does not implement larger than 128-bit keys.

examples/platforms/cc1352/crypto/cc1352-mbedtls-config.h

@@ -31,4 +31,7 @@
 
 #define MBEDTLS_AES_ALT
 
+/* TODO: remove once AES alt driver supports 256-bit keys */
+#define MBEDTLS_CTR_DRBG_USE_128_BIT_KEY
+
 #endif // CC1352_MBEDTLS_CONFIG_H

examples/platforms/cc2650/crypto/cc2650-mbedtls-config.h

@@ -32,4 +32,6 @@
 #define MBEDTLS_AES_ALT
 #define MBEDTLS_SHA256_ALT
 
+#define MBEDTLS_CTR_DRBG_USE_128_BIT_KEY
+
 #endif // CC2650_MBEDTLS_CONFIG_H

examples/platforms/cc2652/crypto/cc2652-mbedtls-config.h

@@ -31,4 +31,7 @@
 
 #define MBEDTLS_AES_ALT
 
+/* TODO: remove once AES alt driver supports 256-bit keys */
+#define MBEDTLS_CTR_DRBG_USE_128_BIT_KEY
+
 #endif // CC2652_MBEDTLS_CONFIG_H