ARM: dts: dra7-evm: add gpio leds support

dra7-evm has 4 user gpio leds connected to PCF8575 GPIO pcf_lcd:
gpio@20 expander pins [4,5,6,7], so add corresponding DT nodes.

Do not enable any triggers by default as not all of them are proved
to work on -RT.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>