kirkwood-kernel: add spi support

add spi support for device MX25L3206E

Signed-off-by: Eik Binschek <binschek@dresearch-fe.de>
kirkwood-kernel: faster spi

change max spi clock from 20MHz to 80MHz

Signed-off-by: Eik Binschek <binschek@dresearch-fe.de>