I used the Marvell git branch for 2.6.30-rc3 & don't recall having to do anything special. I.e., after installing the git package:
# git clone git://git.marvell.com/orion.git
That gets the latest from Marvell.
I installed the cross-compiling tools from codesourcery:http://www.codesourcery.com/sgpp
Then it's the usual:
# make ARCH=arm mrproper
# make ARCH=arm kirkwood_defconfig
# make ARCH=arm menuconfig
# make -j1 ARCH=arm CROSS_COMPILE=/usr/local/bin/codesourcery/bin/arm-none-linux-gnueabi- uImage
When the kernel is booting you should see these messages on the console:
cpuidle: using governor ladder
cpuidle: using governor menu
Just in case I'm forgetting some config option that has to be made, for reference I attached a copy of the .config file I built with.
P.S. The powertop FAQ lists some recommended kernel config settings: http://www.lesswatts.org/projects/powertop/faq.php