tests: drivers: flash stm32l4 RDP on internal flash partition

[FRASTM]

Define the storage_partition in the internal flash instead of the external NOR qspi flash,
so the testcase can PASS on the disco_l4754_iot1 board.
Impact on stm32 CI

Running the west build -p auto -b disco_l475_iot1 tests/drivers/flash/stm32/
with CONFIG_FLASH_STM32_READOUT_PROTECTION=y
It fails taking the storage_partition of the external qspi NOR

Running TESTSUITE flash_stm32
===================================================================
START - test_stm32_readout_protection_disabled

    Assertion failed at WEST_TOPDIR/zephyr/tests/drivers/flash/stm32/src/main.c:174: flash_stm32_tes)
Failed to get RDP status
 FAIL - test_stm32_readout_protection_disabled in 0.017 seconds
===================================================================
TESTSUITE flash_stm32 failed.

------ TESTSUITE SUMMARY START ------

SUITE FAIL -   0.00% [flash_stm32]: pass = 0, fail = 1, skip = 0, total = 1 duration = 0.017 seconds
 - FAIL - [flash_stm32.test_stm32_readout_protection_disabled] duration = 0.017 seconds

------ TESTSUITE SUMMARY END ------

The disco_l475_iot1 partitionning is set by the #30864

[FRASTM]

Running the west build -p auto -b disco_l475_iot1 tests/drivers/flash/stm32/
with CONFIG_FLASH_STM32_READOUT_PROTECTION=y
It takes the storage_partition of the internal flash :

*** Booting Zephyr OS build zephyr-v3.5.0-663-gbc43e890fd7a ***
Running TESTSUITE flash_stm32
===================================================================
START - test_stm32_readout_protection_disabled
RDP is disabled
 PASS - test_stm32_readout_protection_disabled in 0.002 seconds
===================================================================
TESTSUITE flash_stm32 succeeded

------ TESTSUITE SUMMARY START ------

SUITE PASS - 100.00% [flash_stm32]: pass = 1, fail = 0, skip = 0, total = 1 duration = 0.002 seconds
 - PASS - [flash_stm32.test_stm32_readout_protection_disabled] duration = 0.002 seconds

------ TESTSUITE SUMMARY END ------

===================================================================
PROJECT EXECUTION SUCCESSFUL