zephyr: fix jdata pointer
zephyr: fix jdata pointer

Zephyr stores the panic date not at the end of RAM, thus the pointer to
the jump data has to be calculated in a different way.

With the change, the function system_usable_ram_end returns a correct
value which is used to calculate the SharedMem size.

BUG=b:178011288
BRANCH=none
TEST=run firmware_ECSharedMem test on Lazor

Signed-off-by: Dawid Niedzwiecki <73bc8c219b84b29e82d4338ec2e55b540c845bc6@semihalf.com>
Change-Id: If777261a5582071822a998e796973ec5b8cb3cac
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2960174
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>

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