zephyr: provide a header for cpu.h
zephyr: provide a header for cpu.h

Provide a minimal header for cpu.h, which is used by both
panic_output.c and system.c.

BUG=b:174481378,b:178011288
BRANCH=none
TEST=compiles with panic_output.c

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: I16ca7350d58bc392d87a6f0bbf9d6080e607998d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2647951
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