riscv: dts: add fu740 support

Add dts support for fu740. The HiFive Unmatched support is based on
fu740 cpu and drivers in following patch set.

Signed-off-by: Green Wan <green.wan@sifive.com>
[greentime.hu: set fu740 speed to 1.2GHz]
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>