stm32: support i2c3 properly

I2C3 is on many parts, but wasn't properly supported with the register
definitions.  Declare them centrally, just depending on the memorymap
defining them. On some parts, the rcc bits were defined, but not the
base registers.

Fixes: https://github.com/libopencm3/libopencm3/issues/820

include/libopencm3/stm32/l4/i2c.h

@@ -35,8 +35,6 @@ LGPL License Terms @ref lgpl_license
 
 /**@{*/
 
-#define I2C3				I2C3_BASE
-
 /**@}*/
 
 #endif