x86: Add MultiProcessor (MP) table APIs

The MP table provides a way for the operating system to support
for symmetric multiprocessing as well as symmetric I/O interrupt
handling with the local APIC and I/O APIC. We provide a bunch of
APIs for U-Boot to write the floating table, configuration table
header as well as base and extended table entries.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>