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>
