drivers/flash_simulator: Rename FLASH() macro to MOCK_FLASH()
drivers/flash_simulator: Rename FLASH() macro to MOCK_FLASH()

The change is done to avoid redefinition errors while attempting
to enable flash_simulator with stm32 platform (stm32f4_disco)
which exports its own definition of FLASH macro, from stm32f407xx.h.
Unfortunately the stm32 definition is visible within flash_simulator
via inclusion of device.h.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr