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

dra7-evm has 2 gpio keys wired through TS_LCD_GPIO3, TS_LCD_GPIO4
which in turn connected to PCF8575 GPIO pcf_lcd: gpio@20 expander
pins 2 and 3.

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