Add pinmux configurations for gpio volume keys. 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>