Add basic ADC support

Planning on abstracting this behind hardware setup APIs, but this works
for a simple demo until then

CMakeLists.txt

@@ -9,10 +9,12 @@ target_sources(app PRIVATE
     core/main.c
     core/ble.c
     core/routine.c
+    core/hardware/adc.c
 )
 
 # User routines to compile
 target_sources(app PRIVATE
     routines/routine_list.c
     routines/input_echo.c
+    routines/photosensor.c
 )