ARM: OMAP2+: fix CONFIG_CPU_IDLE dependency on CONFIG_PM

commit 164e0cbf60 (ARM: OMAP3/4: consolidate cpuidle Makefile) added
an OMAP-specific dependency from CPU_IDLE to CONFIG_PM.  This causes
some randconfig warnings when CONFIG_PM has unmet dependencies:

warning: (ARCH_OMAP3 && ARCH_OMAP4) selects PM which has unmet direct dependencies (PM_SLEEP || PM_RUNTIME)
warning: (ARCH_OMAP3 && ARCH_OMAP4) selects PM which has unmet direct dependencies (PM_SLEEP || PM_RUNTIME)
warning: (ARCH_OMAP3 && ARCH_OMAP4) selects PM which has unmet direct dependencies (PM_SLEEP || PM_RUNTIME)

Fix this by making the dependency on CONFIG_PM_RUNTIME (which in turn
will enable CONFIG_PM.)

Reported-by: Tony Lindgren <tony@atomide.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>