kw40z: Add kw40z SoC

Adds initial support for the kw40z SoC. This SoC has all the same
peripherals as the kw41z but with less flash and ram, so the defconfig
and dts are nearly the same.

Jira: ZEP-1388
Change-Id: Ib804451e8c2c71c4ff7d342bf23f6567d1542a2d
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>