Backlight - Carve out a better location for private driver functionality (#8329)

* rename backlight_soft to match rules.mk

* rename backlight_soft to match rules.mk - update common_features

* Carve out a better location for private driver backlight functionality

quantum/backlight/backlight_avr.c

@@ -1,11 +1,8 @@
 #include "quantum.h"
 #include "backlight.h"
+#include "backlight_driver_common.h"
 #include "debug.h"
 
-#if !defined(BACKLIGHT_PIN) && !defined(BACKLIGHT_PINS)
-#    error "Backlight pin/pins not defined. Please configure."
-#endif
-
 // This logic is a bit complex, we support 3 setups:
 //
 //   1. Hardware PWM when backlight is wired to a PWM pin.