ARM: dts: am437x-gp-evm: Add uart0 pinctrl default and sleep states

Currently uart0 uses pinctrl config set by bootloader so
create default state that can be restored after a suspend
event.

Also, modify uart0 pinctrl to include RTS and CTS pins as by
default these are not in a mode for optimal power savings.

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