drivers: dma: silabs: add dma block splitting for high transfer size

The LDMA driver currently has a 1:1 mapping between hardware LDMA
descriptors and struct dma_block_config. This patch allows multiple
hardware descriptors to be allocated for a single struct dma_block_config
if the block size exceeds the transfer capacity of a single hardware
LDMA descriptor. This is beneficial for other peripheral drivers: it is
no longer necessary to split the payload by the transfer capacity of a
single hardware LDMA descriptor.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>