clocksource/drivers/timer-ti-dm: Drop bogus omap_dm_timer_of_set_source()

The function omap_dm_timer_of_set_source() was originally added in
commit 31a7448f4fa8a ("ARM: OMAP: dmtimer: Add clock source from DT"),
and is designed to set a clock source from DT using the clocks property
of a timer node. This design choice is okay for clk provider nodes but
otherwise is a bad design as typically the clocks property is used to
specify the functional clocks for a device, and not its parents.

The timer nodes now all define a timer functional clock after the
conversion to ti-sysc and the new clkctrl layout, and this results
in an attempt to set the same functional clock as its parent when a
consumer driver attempts to acquire any of these timers in the
omap_dm_timer_prepare() function. This was masked and worked around
in commit 983a5a43ec25 ("clocksource: timer-ti-dm: Fix pwm dmtimer
usage of fck reparenting"). Fix all of this by simply dropping the
entire function.

Any DT configuration of clock sources should be achieved using
assigned-clocks and assigned-clock-parents properties provided
by the Common Clock Framework.

Cc: Tony Lindgren <tony@atomide.com>
Cc: Tero Kristo <t-kristo@ti.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: H. Nikolaus Schaller <hns@goldelico.com>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.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>
Signed-off-by: Suman Anna <s-anna@ti.com>
Tested-by: Lokesh Vutla <lokeshvutla@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200213053504.22638-1-s-anna@ti.com