vmbase: uart: Create abstraction layer over register write

This change creates an abstraction layer over register write accesses
(there are currently no read accesses) for 8250 UART driver. The direct
write volatile access have been replaced with a intermediate trait that
is accepted via newly introduced generic to `Uart`.

Bug: 362733888
Test: m libvmbase pvmfw_bin vmbase_example_bios_bin vmbase_example_kernel_bin
Test: atest vmbase_example.integration_test
Change-Id: I4ff012ac8ab433e82199c9c52f85ab53af205792
diff --git a/libs/libvmbase/src/arch/aarch64.rs b/libs/libvmbase/src/arch/aarch64.rs
index 199423b..05b5566 100644
--- a/libs/libvmbase/src/arch/aarch64.rs
+++ b/libs/libvmbase/src/arch/aarch64.rs
@@ -21,6 +21,7 @@
 pub mod linker;
 pub mod page_table;
 pub mod platform;
+pub mod uart;
 
 /// Reads a value from a system register.
 #[macro_export]