Make system register functions inline assembly

Replace the current out-of-line assembler implementations of
the system register and system instruction operations with
inline assembler.

This enables better compiler optimisation and code generation
when accessing system registers.

Fixes ARM-software/tf-issues#91

Change-Id: I149af3a94e1e5e5140a3e44b9abfc37ba2324476
Merge pull request #130 from athoelke/at/inline-asm-sysreg-v2

Make system register functions inline assembly v2