zephyr: riscv: linker: correct __image_size
zephyr: riscv: linker: correct __image_size

With CL:3088970, the _image_rom_size has been able to provide
correct image size. So configure __image_size = _image_rom_size;

BRANCH=none
BUG=b:195954913
TEST=software sync is successfully.

Signed-off-by: Dino Li <338d523ecf3700bd86eb9c34d220f91911d2a18c@ite.com.tw>

Cq-Depend: chromium:3088970
Change-Id: I23a2f33aa15df7a8469d5591dc8d310d0313be92
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3105912
Tested-by: Dino Li <338d523ecf3700bd86eb9c34d220f91911d2a18c@ite.com.tw>
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Commit-Queue: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>
Commit-Queue: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec