ARM: dts: DRA7: Add timer12 node

Add the DT node for Timer12 present on DRA7 family of
SoCs. Timer12 is present in PD_WKUPAON power domain, and
has the same capabilities as the other timers, except for
the fact that it serves as a secure timer on HS devices
and is clocked only from the secure 32K clock.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>