x86, platform: Export x86_platform to modules

Export x86_platform to modules in preparation of using it for i8042
discovery control.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
LKML-Reference: <1278342202-10973-1-git-send-email-feng.tang@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Dmitry Torokhov <dtor@mail.ru>