x86: Add functions to convert between mtrr size and mask

x86: Add functions to convert between mtrr size and mask

Rather than repeating the same code in several places, add some
functions which can do the conversion.

Use the cpu_phys_address_size() function to obtain the physical-address
size, since it is more reliable with kvm, where the host CPU may have a
different value from the emulation CPU.

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