sd-image-aarch64: Enable arm_boost for Pi 4

This is effective on the Pi 400 and on newer board revisions of the
Pi 4B. From the official documentation:

> New Raspberry Pi OS images from Bullseye onwards come with
> this setting by default.

<https://www.raspberrypi.com/documentation/computers/config_txt.html>

nixos/modules/installer/sd-card/sd-image-aarch64.nix

@@ -36,6 +36,9 @@
         # what the pi3 firmware does by default.
         disable_overscan=1
 
+        # Supported in newer board revisions
+        arm_boost=1
+
         [all]
         # Boot in 64-bit mode.
         arm_64bit=1