arch: riscv: linker script: add support for rom_start section

With this change, we can put contents into rom_start section
by calling zephyr_linker_sources(ROM_START ...)

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>
Co-authored-by: Torsten Tejlmand Rasmussen
Change-Id: If1169423b013d3e4df52d91cdb2fbdddc3bace7b