riscv: linker: add support for FLASH_LOAD_OFFSET

This allows board to re-configure ROM_BASE address while building
firmware image.

For example, a board need to build two firmware images. And the images
are put into one flash and required to work independent:

config FLASH_LOAD_OFFSET
    default 0 if FIRST_IMAGE
    default 0x80000 if SECOND_IMAGE

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>