riscv: Add missing sentinel in ocores_i2c.c

The ocores_i2c.c driver is missing a sentinel at the end of
the compatible strings list.  This causes the "dm compat" command
to spew garbage.

Signed-off-by: Thomas Skibo <thomas-git@skibo.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>