pid: add helper methods for thermal controller types

Add helper methods for thermal controller types, such as checking if a
string is a supported thermal type and which type that string
corresponds to.

Change-Id: I3c782f5825e7726f264dd186df583bde0a0dc861
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/pid/builder.cpp b/pid/builder.cpp
index 261a165..8fef528 100644
--- a/pid/builder.cpp
+++ b/pid/builder.cpp
@@ -94,7 +94,7 @@
                                                        info->pidInfo);
                 zone->addFanPID(std::move(pid));
             }
-            else if (info->type == "temp" || info->type == "margin")
+            else if (isThermalType(info->type))
             {
                 for (const auto& i : info->inputs)
                 {
@@ -102,19 +102,9 @@
                     zone->addThermalInput(i);
                 }
 
-                ThermalType type;
-                if (info->type == "temp")
-                {
-                    type = ThermalType::absolute;
-                }
-                else
-                {
-                    type = ThermalType::margin;
-                }
-
                 auto pid = ThermalController::createThermalPid(
                     zone.get(), name, inputs, info->setpoint, info->pidInfo,
-                    type);
+                    getThermalType(info->type));
 
                 zone->addThermalPID(std::move(pid));
             }