arch: arm: mpu: move MPU memory configuration header

Move the helper file arm_mpu_mem_cfg.h, which translates the values
of the Kconfig symbols CONFIG_FLASH_SIZE and CONFIG_SRAM_SIZE to size
definitions to be used during MPU setup, from a Cortex-M-specific
include directory to the Cortex-agnostic zephyr/arch/arm/mpu directory,
as:

- the contents of this header file are not dependent on the target being
  an ARMv7M, only ARMv8M is excluded, but the contents are identical
  for the ARMv7/8 Cortex-R MPU implementation,
- the header file zephyr/arch/arm/mpu/arm_mpu.h, included within
  arm_mpu_mem_cfg.h, is also compatible with both Cortex-M and Cortex-R,
  the distinction between the two implementations takes place at an
  even lower level,
- several ARMv7/8 Cortex-R targets now reference this header file in their
  MPU region setup (Xilinx ZynqMP, TI K3, Renode cortex_r8_virtual) while
  so far referencing it in an ARMv7 Cortex-M-specific include directory.

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