CMake: fix incorrect relative path for MBED_PATH

MBED_PATH set to /platform directory incorrectly. Updated to set to
/mbed-os directory.

Author: hazzlim

hal/tests/TESTS/mbed_hal/flash/functional_tests/CMakeLists.txt

@@ -3,7 +3,7 @@
 
 cmake_minimum_required(VERSION 3.19.0 FATAL_ERROR)
 
-set(MBED_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../../../../.. CACHE INTERNAL "")
+set(MBED_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../../../../../.. CACHE INTERNAL "")
 set(TEST_TARGET mbed-hal-flash-functional-tests)
 
 include(${MBED_PATH}/tools/cmake/mbed_greentea.cmake)