tests: stream_flash: Convert old DT syntax to new macros

#define MAX_PAGE_SIZE 0x1000 /* Max supported page size to run test on */

#define MAX_NUM_PAGES 4      /* Max number of pages used in these tests */

#define TESTBUF_SIZE (MAX_PAGE_SIZE * MAX_NUM_PAGES)

#define FLASH_SIZE DT_SOC_NV_FLASH_0_SIZE

#define SOC_NV_FLASH_NODE DT_INST(0, soc_nv_flash)

#define FLASH_SIZE DT_REG_SIZE(SOC_NV_FLASH_NODE)

#define FLASH_NAME DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL

/* so that we don't overwrite the application when running on hw */