sheevaplug_defconfig: use 3.5.4 kernel

Notice that you need u-boot >= 2012.04 to be able to boot kernel >= 3.2,
because older u-boots keep the l2 cache enabled, see u-boot commit
679530278 (arm, arm-kirkwood: disable l2c before linux boot) for details.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>