aarch64: Rename RAM_64BIT_START in layout

`RAM_64BIT_START` was set to 1 GiB, not a real 64-bit address. Now
rename it `RAM_START` to avoid confusion.

Signed-off-by: Michael Zhao <michael.zhao@arm.com>

vmm/src/vm.rs

@@ -2993,7 +2993,7 @@ mod tests {
     #[test]
     fn test_create_fdt_with_devices() {
         let regions = vec![(
-            GuestAddress(layout::RAM_64BIT_START),
+            GuestAddress(layout::RAM_START),
             (layout::FDT_MAX_SIZE + 0x1000) as usize,
         )];
         let mem = GuestMemoryMmap::from_ranges(&regions).expect("Cannot initialize memory");