dmaengine: add driver for lpc18xx dmamux

Add support for DMA on NXP LPC18xx/43xx platforms which has
a multiplexer in front of the PL080 dma request lines.

The mux is a single register in the LPC18xx/43xx CREG block
and can multiplex up to 4 request lines to each of the 16
lines on the PL080.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>