ARM: dts: am335x-boneblack: Use pinctrl constants and AM33XX_IOPAD macro

Using constants for pinctrl allows better readability and removes
redundancy with comments. AM33XX_IOPAD allows us to use part of the
pinctrl physical address as in the TRM instead of an offset.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>