ram: sifive: Avoid using hardcoded ram base and size

At present the SiFive FU540 RAM driver uses hard-coded memory base
address and size to initialize the DDR controller. This may not be
true when this driver is used on another board based on FU540.

Update the driver to read the memory information from DT and use
that during the initialization.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Leo Liang <ycliang@andestech.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>