rsa.c: provide interface to get padding mode of RSA context

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/library/rsa.c b/library/rsa.c
index df7d797..ba54d25 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -500,9 +500,16 @@
 }
 
 /*
+ * Get padding mode of RSA modulus
+ */
+int mbedtls_rsa_get_padding_mode(const mbedtls_rsa_context *ctx)
+{
+    return ctx->MBEDTLS_PRIVATE(padding);
+}
+
+/*
  * Get length in bytes of RSA modulus
  */
-
 size_t mbedtls_rsa_get_len(const mbedtls_rsa_context *ctx)
 {
     return ctx->len;