arm: kirkwood: sheevaplug: reduce u-boot image size

Sheevaplug board has 512K CONFIG_BOARD_SIZE_LIMIT. Recently, DM_SERIAL has
pushed the image size a few hundred bytes over that limit. So explicitly
deselect some configs that are unrelated to this board
(and gain back a bit over 2K).

Signed-off-by: Tony Dinh <mibodhi@gmail.com>