feat(stm32mp2): enable DDR sub-system clock

Create a DDR helper files, and add a function to enable DDR clocks
in RCC_DDRCPCFGR register.
Call this ddr_sub_system_clk_init() just before clock driver init,
as it needs to be done before enabling DDR PLL clock (PLL2).

Change-Id: I365d6aa034363d0c036ce2d9f944f077ba86e193
Signed-off-by: Yann Gautier <yann.gautier@st.com>

drivers/st/ddr/stm32mp2_ddr_helpers.c

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2024, STMicroelectronics - All Rights Reserved
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <lib/mmio.h>
+
+#include <platform_def.h>
+
+void ddr_sub_system_clk_init(void)
+{
+	mmio_write_32(stm32mp_rcc_base() + RCC_DDRCPCFGR,
+		      RCC_DDRCPCFGR_DDRCPEN | RCC_DDRCPCFGR_DDRCPLPEN);
+}