soc: xlnx: zynqmp: overhaul MPU regions

Overhaul the MPU region definitions that are being
configured when the MPU is set up:
- drop local attribute definitions in favor of those
  already provided in arm_mpu_v7m.h
- actually tie the RAM region to the device tree
- set up a (potentially overlapping) R/O region for
  .text and .rodata, which hasn't existed so far
- Consider XIP

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>