PM / OPP: rename header to linux/pm_opp.h

Since Operating Performance Points (OPP) functions are specific
to device specific power management, be specific and rename opp.h
to pm_opp.h

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Git-commit: e4db1c74
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[junjiew@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>