Allow FP register context to be optional at build time

CTX_INCLUDE_FPREGS make variable allows us to include or exclude FP
registers from context structure, in case FP is not used by TSPD.

Fixes ARM-software/tf-issues#194

Change-Id: Iee41af382d691340c7ae21830ad1bbf95dad1f4b
Merge pull request #162 from jcastillo-arm/jc/tf-issues/194

Allow FP register context to be optional at build time