x86: Allow adding non-aligned size for MTRR

x86: Allow adding non-aligned size for MTRR

At present mtrr_add_request() requires that the size is a power of two.
This is too limiting for machines with 4GB (or more) of RAM, since they
often must take account of a memory hole at 3GB.

Update the function to automatically deal with an unaligned size, using
more MTRRs as required.

The algorithm is taken from coreboot commit 60bce10750

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