plat/arm/css: Add SID registers for SGx platforms
plat/arm/css: Add SID registers for SGx platforms

Some of the SGx platforms use System Identification (SID) registers
for platform identification. Add support for these registers in css.

Change-Id: If00b18744a31ff2cf14338f18c8c680eb69c9027
Signed-off-by: Chandni Cherukuri <6a0055f6f2cdf3e44f4734e2e90cb32068da3ded@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware