samples: usb: dfu: exclude lpcxpresso55s69_cpu0
samples: usb: dfu: exclude lpcxpresso55s69_cpu0

Exclude lpcxpresso55s69_cpu0 from the USB DFU tests, this triggers a
build assert due to unsupported flash write block size (512 bytes,
mcuboot supports 8 to 32).

Signed-off-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>

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