arch: riscv: linker: fix _image_rom_start symbol
arch: riscv: linker: fix _image_rom_start symbol

Change _image_rom_start symbol from 0 to the beginning of
ROMABLE_REGION.

Signed-off-by: Jim Shu <1635390bb889c4cdee38f53a57b0915ac07d9bd9@andestech.com>

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