x86: Add support for MTRRs

Memory Type Range Registers are used to tell the CPU whether memory is
cacheable and if so the cache write mode to use.

Clean up the existing header file to follow style, and remove the unneeded
code.

These can speed up booting so should be supported. Add these to global_data
so they can be requested while booting. We will apply the changes during
relocation (in a later commit).

Signed-off-by: Simon Glass <sjg@chromium.org>