samples: code relocation for the stm32f746 disco kit

[FRASTM]

Add the configuration for using the external FLASh Memory and execute the samples/application_development/code_relocation_nocopy/ on the stm32f746g_disco board

Fixes #86123

The ext_memory region is declared directly in the stm32f746g_disco.dts
The stm32cubeprogrammer runner includes the corresponding external loader to flash in internal and external memory

--> download the zephyr.hex in internal and external flash (for example : west flash or west flash --runner openocd)

[FRASTM]

[00:00:00.000,000] <inf> flash_stm32_qspi: Reading SFDP
[00:00:00.000,000] <inf> flash_stm32_qspi: qspi-nor-flash@90000000: SFDP v 1.5 AP ff with 2 PH
[00:00:00.000,000] <inf> flash_stm32_qspi: PH0: ff00 rev 1.5: 16 DW @ 30
[00:00:00.000,000] <inf> flash_stm32_qspi: Reading SFDP
[00:00:00.000,000] <inf> flash_stm32_qspi: qspi-nor-flash@90000000: 16 MiBy flash
[00:00:00.000,000] <inf> flash_stm32_qspi: PH1: ff03 rev 1.0: 2 DW @ 100
[00:00:00.000,000] <inf> flash_stm32_qspi: Memory-mapped NOR quad-flash at 0x90000000 (0x1000000 bytes)
*** Booting Zephyr OS build v4.0.0-37-g162d3f4f15b1 ***
Address of main function 0x800059d
Address of function_in_ext_flash 0x90000001
Address of var_ext_sram_data 0x200100e0 (10)
Address of function_in_sram 0x20010001
Address of var_sram_data 0x200100e4 (10)                                                                                   
Hello World! stm32f746g_disco  

[mathieuchopstm]

Do we need this comment? ATTR_MPU_EXTMEM is expected to cause MPU faults:

zephyr/include/zephyr/arch/arm/mpu/arm_mpu_v7m.h

Line 133 in ad845fd

#define REGION_EXTMEM_ATTR(size) {(STRONGLY_ORDERED_SHAREABLE | size | NO_ACCESS_Msk)}

(Emphasis mine: | NO_ACCESS_Msk)