ARM: dts: DRA7: provide arch-timer frequenecy parameter

CNTFREQ isn't pre-programmed on DRA7 just like O5, so provide the
timer frequency via DT. Without a valid value arch_timer_init results
in div0 crash.

Cc: R Sricharan <r.sricharan@ti.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Sourav Poddar <sourav.poddar@ti.com>

Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>