ARM: dts: am437x-gp-evm: Add pinmux for gpio0 wake

Add pinctrl settings so that gpio0 wake from suspend will be supported
using buttons SW4 and SW7. Also, add pinctrl configuration for 0x954,
spi0_d0, which is an unused pin brought out to a header on the board
that in it's default state also connects to the gpio used for wakeup,
gpio0_3, which affects the state of the pin and prevents a working
wakeup unless we set the mux to a different state.

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>