board/raspberrypi/readme.txt: reword overlays note

Any Raspberry Pi may use device-tree overlays; it is not restricted to
Raspberry Pi 3 and 4.

The defconfigs for the Raspberry Pi 3, 4, 400, Zero W, Zero 2 W, Compute
Module 4 and 4s use the miniuart-bt dtoverlay to switch the Bluetooth to
the mini-UART (ttyS0) and restore UART0 (ttyAMA0) over GPIOs 14 and 15
to enable the serial console (like the Raspberry Pi 1 and 2 that have no
Bluetooth)[1][2].

As a consequence, the Raspberry Pi Zero W, Zero 2 W, 400, Compute Module
4 and 4s install the device-tree overlays as well.

This rewords the note by adding the three missing hardwares installing
the images/rpi-firmware/overlays directory. It rewords the miniuart note
at the same time.

[1]: https://www.raspberrypi.com/documentation/computers/configuration.html#uarts-and-device-tree
[2]: https://github.com/raspberrypi/linux/blob/rpi-6.6.y/arch/arm/boot/dts/overlays/miniuart-bt-overlay.dts#L4-L6

Signed-off-by: Gaël PORTAY <gael.portay+rtone@gmail.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/raspberrypi/readme.txt

@@ -131,10 +131,11 @@ After building, you should obtain this tree:
     Pi 4, 400, 5 and the Compute Module 4, 4s and 5 load the second stage
     bootloader from a SPI flash EEPROM.
 
-[3] Only for the Raspberry Pi 3/4 Models (overlay miniuart-bt is needed
-    to enable the RPi3 serial console otherwise occupied by the bluetooth
-    chip). Alternative would be to disable the serial console in cmdline.txt
-    and /etc/inittab.
+[3] Only for the Raspberry Pi installing device-tree overlays. The Raspberry Pi
+    with Bluetooth connectivity (Zero W, Zero 2 W, 3, 4, 400, Compute Module 4
+    and 4s) use the miniuart-bt overlay to enable UART0 for the serial console;
+    the Bluetooth uses the mini-UART instead. Alternative would be to disable
+    the serial console in cmdline.txt and /etc/inittab.
 
 How to write the SD card
 ========================