devicetree: make DT_..._REG_ADDR return unsigned

`DT_..._REG_ADDR` macros do not always return an unsigned value. This
commit adds and unsigned 32bit prefix to ensure the value is always
unsigned.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>