imx8mq: fix soc_id issue

The chip revision should ONLY overwrite the lower 16 bits of
soc_id, otherwise, the cpu_is_imx8mq() API in Linux kernel
will be incorrect.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
(cherry picked from commit ca98a9fca54b491755e515ca0f0d7f53a19ad74f)
diff --git a/plat/imx/imx8mq/src.c b/plat/imx/imx8mq/src.c
index c9af655..696f68b 100644
--- a/plat/imx/imx8mq/src.c
+++ b/plat/imx/imx8mq/src.c
@@ -65,7 +65,8 @@
 		} else if (mmio_read_32(IMX_OCOTP_BASE + SW_INFO_B1)
 			   == 0xff0055aa) {
 			/* 0xff0055aa is magic number for B1 */
-			val = 0x21;
+			val &= ~0xff;
+			val |= 0x21;
 		}
 	}