pm: cpupower: Makefile: Allow overriding cross-compiling env params

Allow overriding the cross-comple env parameters to make it
easier for Yocto users. Then cross-compiler toolchains to build
cpupower with only two steps:
- source (toolchain path)/environment-setup-armv8a-poky-linux
- make

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>