Use MRS/MSR instructions in assembler code

This change is based on top of the barriers review change https://github.com/ARM-software/arm-trusted-firmware/pull/52

This changes all usage of the system register access helper function in the assembler code to using the `mrs` and `msr` instructions directly, and then removes the redundant and invalid helper functions from the code.

It partially addresses ARM-software/tf-issues#91