ARM: dts: lpc18xx: configure dwmac properly

Configure the DWMAC FIFO sizes, burst length and threshold DMA mode on
LPC18xx/43xx SoCs. These parameters are needed for correct operation of
the Ethernet MAC. The DWMAC on LPC18xx/43xx does not support RSF DMA
mode and therefore must use threshold mode.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>