update mach-omap2/pm.c for struct subsystem removal

update mach-omap2/pm.c for struct subsystem removal

'struct subsystem' is no more, replaced with 'struct kset': commit
823bccfc4002296ba88c3ad0f049e1abd8108d30.  Fixes the following
warnings:

  CC      arch/arm/mach-omap2/pm.o
arch/arm/mach-omap2/pm.c:410: warning: 'struct subsystem' declared inside parameter list
arch/arm/mach-omap2/pm.c:410: warning: its scope is only this definition or declaration, which is probably not what you want
arch/arm/mach-omap2/pm.c:417: warning: 'struct subsystem' declared inside parameter list
arch/arm/mach-omap2/pm.c:434: warning: initialization from incompatible pointer type
arch/arm/mach-omap2/pm.c:435: warning: initialization from incompatible pointer type

Compile-tested only.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>