backports: backport some memory functions

backports: backport some memory functions

This includes the following functions needed by some drm drivers:
arch_phys_wc_add()
arch_phys_wc_del()
phys_wc_to_mtrr_index()

This backports the following commit form mainline kernel:
commit d0d98eedee2178c803dd824bb09f52b0e2ac1811
Author: Andy Lutomirski <luto@amacapital.net>
Date:   Mon May 13 23:58:40 2013 +0000

    Add arch_phys_wc_{add, del} to manipulate WC MTRRs if needed

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>