dts: bindings: Rename nxp,kinetis-lptmr compat

Rename nxp,kinetis-lptmr compat to nxp,lptmr.
Because of concerns over breaking downstream users,
keep support for the old compatible temporarily and
make it clear it should be changed.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>