ARM: dts: am335x-icev2: Add nodes for gpio-decoder

AM335x ICE board has a rotary-switch connected to PCA9536 I2C GPIO
expander. The position of the rotary-switch is reflected by status of
GPIO lines. Add gpio-decoder node to read these GPIO line status via
gpio-decoder driver and report it as an input event to the system.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>