drivers: spi_flash_at45: Replace hardcoded sector size
drivers: spi_flash_at45: Replace hardcoded sector size

Replace hardcoded sector size value in sample with sector-size
property added in dts binding file

Signed-off-by: Marin Jurjevic <b0cd402275f93db923f965efd471b503030e79b6@hotmail.com>

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