riscv: add iicbus and iicoc to GENERIC

The iicoc driver supports the OpenCores I2C IP. This is included in at
least the SiFive "Unleashed" and "Unmatched" cores and probably others.

Suggested by: jrtc27

(cherry picked from commit 91feb4f42075e2d7c8d6d50cd5faa651ab3a0e7d)