dmaengine i.MX sdma: check sg entries for valid addresses and lengths

This patch lets sdma_prep_slave_sg fail if the entries of an
sg list do not start on multiples of the word size or if the
lengths are not multiple of the word size.
Also, catch the previously unhandled DMA_SLAVE_BUSWIDTH_8_BYTES
and DMA_SLAVE_BUSWIDTH_UNDEFINED cases.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>