docs(rme): minor update to 4 world execution instructions

Following updates done
  - Clarification on building Hafnium
  - New test suite "Invalid memory access"

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I04a934a648d53a860f06cd6cf3776ee534675bd9

docs/components/realm-management-extension.rst

@@ -154,6 +154,8 @@ You can find instructions on how to get and build TF-A Tests above.
 
  git clone --recurse-submodules https://git.trustedfirmware.org/hafnium/hafnium.git
  cd hafnium
+ #  Use the default prebuilt LLVM/clang toolchain
+ PATH=$PWD/prebuilts/linux-x64/clang/bin:$PWD/prebuilts/linux-x64/dtc:$PATH
  make PROJECT=reference
 
 The Hafnium binary should be located at
@@ -254,6 +256,8 @@ The bottom of the output from *uart0* should look something like the following.
                                                                 Passed
  > Test suite 'Realm payload tests'
                                                                 Passed
+ > Test suite 'Invalid memory access'
+                                                                Passed
  ...