include: Add nomips16 CPU_SUBTYPE

This can be used to indicate that a target does not support the optional mips16
extension even when it is a mips32r2 or later CPU.

This will generate a separate toolchain and a separate package folder,
e.g. mips_24kc_nomips16

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>