ARM: dts: bcm2711: Allow 40-bit DMA for SPI

Add the spi_dma4 DT parameter to enable use of the 40-bit DMA channels
to drive SPI. Note that there are only 3-4 40-bit channels available,
and using this parameter claims 2 of them.

Usage: dtparam=spi_dma4

Signed-off-by: Phil Elwell <phil@raspberrypi.com>