zephyr: cros_system_npcx: do jtag_init only when dbg node exists
zephyr: cros_system_npcx: do jtag_init only when dbg node exists

With this CL, we can build the zephyr project even when the dbg node
is not declared in the device tree.

BRANCH=none
BUG=none
TEST=remove dbg node from npcx.dtsi, there is no build error reported.

Signed-off-by: Jun Lin <4865d89268068846db8afe2dcacc793288c792a0@nuvoton.com>
Change-Id: I8c4ec1f81ba04e62d9b871b10d185d1244a2da6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2918808
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Commit-Queue: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Tested-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>

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