imx8: Replace SC_R_LAST with SC_R_NONE in DTB

We are currently using SC_R_LAST as a marker for imx8 power domain tree
nodes without a resource attached. This value is compiled into dtb as
part of the linux build and used by uboot.

The SC_R_LAST constant changes frequently as SCFW resources are added
(by design) and every time we need to update linux and uboot headers
together or boot can fail.

Fix this by replacing SC_R_LAST usage with a new constant SC_R_NONE
defined to be 0xFFF0.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>