riscv: sifive: fu740: Support i2c in spl

Enable SPL_I2C_SUPPORT for fu740, and add 'u-boot,dm-spl' property in
i2c node.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>