ARM: dts: dra72-evm: Add power button node

With Commit adff5962

 ("Input: introduce palmas-pwrbutton"), we can
now support tps power button as a event source - This is SW7 (PB/WAKE)
on the J6-evm.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>