ARM: dts: am437x-gp-evm: Add pinctrl for debugss pins

The pins used by debugss are not configued by default, place pulldowns
on the pins for maximum power savings during sleep.

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>