arch: do not hardcode MMIO region length in MmioDeviceInfo

Add a field for its length and fix up users.

Things work just because all hardcoded values agree with each other.
This is prone to breakage.

No functional change.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

arch/src/lib.rs

@@ -158,6 +158,7 @@ impl fmt::Display for DeviceType {
 #[cfg(target_arch = "aarch64")]
 pub struct MmioDeviceInfo {
     pub addr: u64,
+    pub len: u64,
     pub irq: u32,
 }
 
@@ -180,6 +181,6 @@ impl DeviceInfoForFdt for MmioDeviceInfo {
         self.irq
     }
     fn length(&self) -> u64 {
-        4096
+        self.len
     }
 }