Add additional changes needed to use ARM ADC

mcuconf.h and halconf.h were not mentioned in adc_driver.md; STM32_ADC_USE_ADC1 used for compatibility.

docs/adc_driver.md

@@ -18,6 +18,20 @@ Then place this include at the top of your code:
 #include "analog.h"
 ```
 
+### ARM Usage
+If using an ARM device, you will _also_ need to add the following to `mcuconf.h`:
+
+```c
+#undef STM32_ADC_USE_ADC1
+#define STM32_ADC_USE_ADC1 TRUE
+```
+
+Add the following to `halconf.h`:
+
+```c
+#define HAL_USE_ADC TRUE
+```
+
 ## Channels
 
 ### AVR