ARM: dts: am57xx-beagle-x15: Use DRA7XX_CORE_IOPAD pinmux macro

Use the pinmux IOPAD macro to define the register absolute physical
address instead of the offset from the padconf base address. This
makes the DTS easier to read since matches the addresses listed in
the Technical Reference Manual.
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>