rosenthal: Add u-boot-rpi-arm64.

* rosenthal/packages/bootloaders.scm: New file.
(u-boot-rpi-arm64): New variable.

rosenthal/packages/bootloaders.scm

@@ -0,0 +1,9 @@
+;; SPDX-FileCopyrightText: 2022 Hilton Chain <hako@ultrarare.space>
+;;
+;; SPDX-License-Identifier: GPL-3.0-or-later
+
+(define-module (rosenthal packages bootloaders)
+  #:use-module (gnu packages bootloaders))
+
+(define-public u-boot-rpi-arm64
+  (make-u-boot-package "rpi_arm64" "aarch64-linux-gnu"))