overlays: audremap: Document CM4 40&41 restriction

Update audremap information to state pins 40,41 are not available on the CM4.

Signed-off-by: James Hughes ([email protected])

Author: JamesH65

arch/arm/boot/dts/overlays/README

@@ -703,7 +703,8 @@ Params: swap_lr                 Reverse the channel allocation, which will also
                                 nothing on BCM2711 (default off)
         pins_12_13              Select GPIOs 12 & 13 (default)
         pins_18_19              Select GPIOs 18 & 19
-        pins_40_41              Select GPIOs 40 & 41
+        pins_40_41              Select GPIOs 40 & 41 (not available on CM4, used
+                                for other purposes)
         pins_40_45              Select GPIOs 40 & 45 (don't use on BCM2711 - the
                                 pins are on different controllers)