[STM32L1:ADC] module refinement

 * This request is based on @bgamari's STM32L1 ADC interface pull request #165.
 * Detailed modifications of this request are described in bgamari#2.
 * Both single shot and multiple (scan mode with DMA) regular ADC conversion are successfully tested with real hardware [STM32L152C-DISCOVERY](http://www.st.com/web/catalog/tools/FM116/SC959/SS1532/LN1199/PF258515) evaluation kit. And the related example codes are in [adc.c](https://github.com/cyhuang/libopencm3/blob/master/lib/stm32/l1/adc.c) file's description comments.