Includes a stub .c file until there are some APIs
libopencm3/include/libopencm3/efm32/common/i2c_common.h:234:29: error: 'base' undeclared (first use in this function) #define I2C0_CTRL I2C_CTRL(base)
Somewhat replaces some earlier work done by hg/lg, but much more complete, so we kept it as is, because it's bringing in even more parts after this. Reviewed-by: Karl Palsson <karlp@tweak.net.au>