Inline the mmio accessor functions

Making the simple mmio_read_*() and mmio_write_*() functions inline
saves 360 bytes of code in FVP release build.

Fixes ARM-software/tf-issues#210

Change-Id: I65134f9069f3b2d8821d882daaa5fdfe16355e2f
Merge pull request #154 from athoelke/at/inline-mmio

Inline the mmio accessor functions