sgi/install_prerequisites.sh: install 'cmake' using pip

CMake package provided through apt repository is not compatible with the
SCP firmware's CMake requirements. So install the latest version of
CMake using Package installer for Python (pip).

Signed-off-by: Zakaria Zahi <zakaria.zahi@arm.com>
Change-Id: Ib9e80456f5706fed2f5418f83b4a7d7927a69fe8