ram: k3-ddrss: Set SDRAM_IDX using device private data, ddr_ram_size

ram: k3-ddrss: Set SDRAM_IDX using device private data, ddr_ram_size

The SDRAM_IDX in DDRSS_V2A_CTL_REG describes the number of address bits
minus 16 that are used to determine the mask used to detect memory
rollover and prevent aliasing and false coherency issues.

Set SDRAM_IDX using the device private data, ddr_ram_size for K3 family
of SoCs.

Signed-off-by: Santhosh Kumar K <s-k6@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>