amd: rename model_fxx_powernow to powernow.

Change-Id: Iee581183f9cd9f5fecd5604536b735f6a04a0f93
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/7019
Tested-by: build bot (Jenkins)
Reviewed-by: Rudolf Marek <r.marek@assembler.cz>