ram: stm32mp1_ram: Fix warnings when compiling with W=1

ram: stm32mp1_ram: Fix warnings when compiling with W=1

This patch solves the following warnings:

drivers/ram/stm32mp1/stm32mp1_ram.c: In function 'stm32mp1_ddr_clk_enable':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (idx = 0; idx < ARRAY_SIZE(clkname); idx++) {
                    ^
drivers/ram/stm32mp1/stm32mp1_ram.c: In function 'stm32mp1_ddr_setup':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (idx = 0; idx < ARRAY_SIZE(param); idx++) {
                    ^
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>