boards: hifive_unleashed: switch zephyr SRAM region from DDR to L2LIM

Switch from `ram0` to `l2lim` in `zephyr, sram` in board DTS

Add `l2lim` in `support/hifive_unleashed.resc` and targets l2lim
as a work-area in `openocd_hifive_unleashed.cfg`

Signed-off-by: Jakub Wasilewski <jwasilewski@internships.antmicro.com>
Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>