ARM: dts: am437x-gp-evm: Add pinctrl for unused_pins

There are several pins on this EVM that are not in use but they can
still draw power if misconfigured. Create a pinctrl entry for these pins
and configure each one for optimal power savings.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
[t-kristo@ti.com: converted to use AM4372_IOPAD macro]
Signed-off-by: Tero Kristo <t-kristo@ti.com>

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>