soc: rp2350: add IMAGE_DEF using rom_start linker script

Use this approach for a simpler flow and to make relocation of images
into RAM easier.

Also do not force-select CONFIG_XIP (which is a default anyway), since
RP2350 can boot from SRAM.

Signed-off-by: Dmitrii Sharshakov <d3dx12.xx@gmail.com>