ARM: dts: AM33XX: Add push-buttons device tree data to am335x-evmsk

Add gpio based push buttons device tree data to am335x-evmsk device
by adding all the necessary parameters like key-code, gpios and etc.

Signed-off-by: AnilKumar Ch <anilkumar@ti.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>