samples: flash_shell: Add board to platform_exclude list

Exclude Arduino Portenta H7 because the flash driver isn't
supported yet on the M4 core.

Signed-off-by: Benjamin Björnsson <[email protected]>

Author: benjaminbjornsson

samples/drivers/flash_shell/sample.yaml

@@ -6,6 +6,6 @@ tests:
   sample.drivers.flash.shell:
     tags: flash shell
     filter: CONFIG_FLASH_HAS_DRIVER_ENABLED
-    platform_exclude: nucleo_h745zi_q_m4 stm32h747i_disco_m4 gd32f350r_eval
+    platform_exclude: nucleo_h745zi_q_m4 stm32h747i_disco_m4 gd32f350r_eval arduino_portenta_h7_m4
     harness: keyboard
     min_ram: 12