ARM: dts: dra7-evm: add pcf8575 gpio expander (i2c1 addr 20)

This patch adds DT definition for CF8575 GPIO pcf_lcd: gpio@20
expander which is connected to i2c bus 1 and has slave address 0x20.
It allows to control:
- tc_lcd gpios, pins p0-p3
- user leds, pins p4-p7
- control LCD panel power, p15

PCF8575 GPIO pcf_lcd: gpio@20 expander supports interrupt controller
functionality and its INT line is connected to dra7 GPIO6.11 pin.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>