Docs: Update GettingStarted and DockerFile

Updated prerequisites to install python3-pip and then use pip3 to
install fdt that is required by hftest.py in both GettingStarted and
Dockerfile.

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@icloud.com>
Change-Id: I58f0ba1e43b1d60eaff2135009a525c9ac35748b
diff --git a/docs/GettingStarted.md b/docs/GettingStarted.md
index a1d53b7..4fb870e 100644
--- a/docs/GettingStarted.md
+++ b/docs/GettingStarted.md
@@ -24,7 +24,8 @@
 Install prerequisites:
 
 ```shell
-sudo apt install make libssl-dev flex bison python3 python3-serial
+sudo apt install make libssl-dev flex bison python3 python3-serial python3-pip
+pip3 install fdt
 ```
 
 By default, the hypervisor is built with clang for a few target platforms along