boards: arm: mps2_an521: Clarify memory map
boards: arm: mps2_an521: Clarify memory map

Attempts to clarify the memory map on the MPS2 AN521, showing how
the 4MB code region and 4MB SRAM region are partitioned across the
various board targets.

Signed-off-by: Kevin Townsend <27ea6d5f9c761dd198db7492b3cb9d20ef0e1d9c@linaro.org>

zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr