ARM: dts: ti81xx: Add dts boilerplate for all GPIO and SPI peripherals

GPIO3/4 and MCSPI2/3/4 are now present. Lightly tested on am3874
platform.

Signed-off-by: Graeme Smecher <gsmecher@threespeedlogic.com>
[tony@atomide.com: split to apply hwmod and dts changes separately]
Signed-off-by: Tony Lindgren <tony@atomide.com>