Use safe-mmio crate for PhysicalInstance.

Change-Id: Id34d4ee2f7bc6ce155a0b5c8aec2f808148ef7f0
Signed-off-by: Andrew Walbran <qwandor@google.com>

diff --git a/Cargo.lock b/Cargo.lock
index 9d29226..1419f48 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -9,6 +9,7 @@
  "arm-gic",
  "arm-pl011-uart",
  "arm-sp805",
+ "safe-mmio",
  "spin",
 ]
 
@@ -94,6 +95,12 @@
 ]
 
 [[package]]
+name = "safe-mmio"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "30075950f15d2083592cdb9b5b175d48d332dacdc481c17546910e0c1e9cc4b1"
+
+[[package]]
 name = "spin"
 version = "0.9.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"