timer: deprecate phosphor namespace

"phosphor" is a namespace used in some OpenBMC repositories but has
only one usage in sdbusplus and the class is not very OpenBMC-specific.

Move the Timer class into the sdbusplus namespace and mark
'phosphor::Timer' as a deprecated alias.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia8c79a25755c9a2c4dc6f92bf1de8100622ef313
diff --git a/example/asio-example.cpp b/example/asio-example.cpp
index 474d261..08fcd11 100644
--- a/example/asio-example.cpp
+++ b/example/asio-example.cpp
@@ -362,9 +362,9 @@
         "/xyz/openbmc_project/sensors", depth, interfaces);
 
     // sd_events work too using the default event loop
-    phosphor::Timer t1([]() { std::cerr << "*** tock ***\n"; });
+    sdbusplus::Timer t1([]() { std::cerr << "*** tock ***\n"; });
     t1.start(std::chrono::microseconds(1000000));
-    phosphor::Timer t2([]() { std::cerr << "*** tick ***\n"; });
+    sdbusplus::Timer t2([]() { std::cerr << "*** tick ***\n"; });
     t2.start(std::chrono::microseconds(500000), true);
     // add the sd_event wrapper to the io object
     sdbusplus::asio::sd_event_wrapper sdEvents(io);