riscv: sifive: dts: fu540: Add board -u-boot.dtsi files

riscv: sifive: dts: fu540: Add board -u-boot.dtsi files

Devicetree files in FU540 platform is synced from Linux, like other
platforms does. Apart from these U-Boot in FU540 would also require
some U-Boot specific node like clint.

So, create board specific -u-boot.dtsi files. This would help of
maintain U-Boot specific changes separately without touching Linux
dts(i) files which indeed easy for syncing from Linux between
releases.

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