ARM: OMAP2+: AM43x: basic dt support

Describe minimal DT boot machine details for AM43x based SoC's. AM43x
SoC's are ARM Cortex-A9 based with one core. AM43x is similar to
AM335x w.r.t L4 PER/WKUP memory map. AM43x has a sync timer, here that
is being used as clocksource, while 1ms dmtimer as clockevent.

Signed-off-by: Ankur Kishore <a-kishore@ti.com>
Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>