vmbase: aarch64: Move power to platform

This commit add new directory `arch` that is intended to hold all
platform specific source code. Additionally, it moves functions `reboot`, `shutdown` the newly created directory.

Bug: 362733888
Test: m libvmbase

Change-Id: I515407e2fa55603c07651ef5d85c8c782f5307cd
diff --git a/libs/libvmbase/src/arch/aarch64.rs b/libs/libvmbase/src/arch/aarch64.rs
index 5006aca..068e001 100644
--- a/libs/libvmbase/src/arch/aarch64.rs
+++ b/libs/libvmbase/src/arch/aarch64.rs
@@ -14,6 +14,8 @@
 
 //! Wrappers of assembly calls.
 
+pub mod platform;
+
 /// Reads a value from a system register.
 #[macro_export]
 macro_rules! read_sysreg {