Add pinmux configurations for gpio based user-keys to am335x-bone.
In this patch, only single named mode/state is added and these pins
are configured during pinctrl driver initialization.

Default mode is nothing but the values required for the module during
active state. With this configurations module is functional as
expected.
Signed-off-by: AnilKumar Ch <anilkumar@ti.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>