rex: update zephyr board to correct version for P0
rex: update zephyr board to correct version for P0

This CL sets the zephyr_board to the correct value for the EC. This
version has 384kB flash space for both RO/RW instead of 256kB.

BRANCH=none
BUG=b:240434243
TEST=zmake build rex

Signed-off-by: Scott Collyer <7d40b3eb9e17bf55d972074a5e77a7ea4159f9dd@google.com>
Change-Id: I7a1bb5b33a8915a931dfb2aa1f4fac0ac8cbf8af
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3808540
Reviewed-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>
Commit-Queue: Scott Collyer <7d40b3eb9e17bf55d972074a5e77a7ea4159f9dd@chromium.org>
Tested-by: Scott Collyer <7d40b3eb9e17bf55d972074a5e77a7ea4159f9dd@chromium.org>

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