soc/sifive-freedom: Configure SoC for I2C driver

Configure the SiFive Freedom SoC to bind to the I2C Peripheral.

Note: The FE310-G000 used on the HiFive 1 does not posess the I2C
peripheral. However, the FE310 platform can have an I2C device memory
mapped in the location described in riscv32-fe310.dtsi.

Signed-off-by: Nathaniel Graff <nathaniel.graff@sifive.com>