sifive: dts: fu540: Add DDR controller and phy register settings

Add DDR controller and phy register settings, taken from fsbl
(https://github.com/sifive/freedom-u540-c000-bootloader.git

)

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>