ARM: OMAP1: fix build breakage introduced by commit 25c7d49e

Commit 25c7d49e ("ARM: OMAP: Make
omap_device local to mach-omap2") broke an OMAP5912-only build here:

arch/arm/mach-omap1/pm_bus.c: In function 'omap1_pm_runtime_init':
arch/arm/mach-omap1/pm_bus.c:69:2: error: implicit declaration of function
'cpu_class_is_omap1'
make[1]: *** [arch/arm/mach-omap1/pm_bus.o] Error 1

Fix by adding a missing include.

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