crypto: mxs - Fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/crypto/mxs-dcp.c:103:1: warning:
 symbol 'global_mutex' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: NMarek Vasut <marex@denx.de>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/mxs-dcp.c

@@ -100,7 +100,7 @@ struct dcp_sha_req_ctx {
  * design of Linux Crypto API.
  */
 static struct dcp *global_sdcp;
-DEFINE_MUTEX(global_mutex);
+static DEFINE_MUTEX(global_mutex);
 
 /* DCP register layout. */
 #define MXS_DCP_CTRL				0x00