clocksource: timer-ti-dm: Fix pwm dmtimer usage of fck reparenting

clocksource: timer-ti-dm: Fix pwm dmtimer usage of fck reparenting

Commit 84badc5ec5fc ("ARM: dts: omap4: Move l4 child devices to probe
them with ti-sysc") moved some omap4 timers to probe with ti-sysc
interconnect target module. Turns out this broke pwm-omap-dmtimer
where we now try to reparent the clock to itself with the following:

omap_dm_timer_of_set_source: failed to set parent

With ti-sysc, we can now configure the clock sources in the dts
with assigned-clocks and assigned-clock-parents. So we should be able
to remove omap_dm_timer_of_set_source with clean-up patches later on.
But for now, let's just fix it first by checking if parent and fck
are the same and bail out of so.

Fixes: 84badc5ec5fc ("ARM: dts: omap4: Move l4 child devices to probe them with ti-sysc")
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: H. Nikolaus Schaller <hns@goldelico.com>
Cc: Keerthy <j-keerthy@ti.com>
Cc: Ladislav Michl <ladis@linux-mips.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Tero Kristo <t-kristo@ti.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-by: H. Nikolaus Schaller <hns@goldelico.com>
Tested-By: Andreas Kemnade <andreas@kemnade.info>
Tested-By: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>