ddr: stm32mp1_ddr: correct SELFREF_TO_X32 mask

In DDR controller PWRTMG register, the mask for field SELFREF_TO_X32 is
wrong. This field is from bit 16 to 23.

Change-Id: Id336fb08c88f0a153df186dd819e41af72febb88
Signed-off-by: Yann Gautier <yann.gautier@st.com>

include/drivers/st/stm32mp1_ddr_regs.h

 /*
- * Copyright (c) 2017-2019, STMicroelectronics - All Rights Reserved
+ * Copyright (c) 2017-2021, STMicroelectronics - All Rights Reserved
  *
  * SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause
  */
@@ -284,7 +284,7 @@ struct stm32mp1_ddrphy {
 #define DDRCTRL_PWRCTL_EN_DFI_DRAM_CLK_DISABLE	BIT(3)
 #define DDRCTRL_PWRCTL_SELFREF_SW		BIT(5)
 
-#define DDRCTRL_PWRTMG_SELFREF_TO_X32_MASK	GENMASK(19, 12)
+#define DDRCTRL_PWRTMG_SELFREF_TO_X32_MASK	GENMASK(23, 16)
 #define DDRCTRL_PWRTMG_SELFREF_TO_X32_0		BIT(16)
 
 #define DDRCTRL_RFSHCTL3_DIS_AUTO_REFRESH	BIT(0)