ARM: OMAP3/4: consolidate cpuidle Makefile

The current Makefile compiles the cpuidle34xx.c and cpuidle44xx.c files
even if the cpuidle option is not set in the kernel.

This patch fixes this by creating a section in the Makefile where these
files are compiled only if the CONFIG_CPU_IDLE option is set.

This modification breaks an implicit dependency between CPU_IDLE and PM as
they belong to the same block in the Makefile. This is fixed in the Kconfig
by selecting explicitely PM is CPU_IDLE is set.

The linux coding style recommend to use no-op functions in the headers
when the subsystem is disabled instead of adding big section in C files.

This patch fix this also.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Jean Pihet <j-pihet@ti.com>
Reviewed-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>