ARM: dts: Configure system timers for omap5 and dra7

We can now init system timers using the dmtimer and 32k counter
based on only devicetree data and drivers/clocksource timers.
Let's configure the clocksource and clockevent, and drop the old
unused platform data.

As we're just dropping platform data, and the early platform data
init is based on the custom ti,hwmods property, we want to drop
both the platform data and ti,hwmods property in a single patch.

Since the dmtimer can use both 32k clock and system clock as the
source, let's also configure the SoC specific default values. The
board specific dts files can reconfigure these with assigned-clocks
and assigned-clock-parents as needed.

Note that similar to omap_init_time_of(), we now need to call
omap_clk_init() also from omap5_realtime_timer_init().

Cc: devicetree@vger.kernel.org
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Keerthy <j-keerthy@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>