Add support for SSC_VERSION register on CSS platforms

Add support for SSC_VERSION register on CSS platforms

Each ARM Compute Subsystem based platform implements a System Security
Control (SSC) Registers Unit. The SSC_VERSION register inside it carries
information to identify the platform. This enables ARM Trusted Firmware
to compile in support for multiple ARM platforms and choose one at
runtime. This patch adds macros to enable access to this register.
Each platform is expected to export its PART_NUMBER separately.

Additionally, it also adds juno part number.

Change-Id: I2b1d5f5b65a9c7b76c6f64480cc7cf0aef019422